1) it's a system of enculturation – teaching the broad "culture" of human discovery, from the simple discovery of the words we use in speech to the more complex discoveries of what we know about microbiology and theoretical physics and macroeconomics, and how we think about these and other topics. An education introduces the mind to some portion of all the findings and thoughts that have been found and thought before us. A formal education does this in the context of a school, where decisions have been made about which topics will be presented, and which won't. Higher education – the thing taking place in colleges and universities – is education at a complexity level that presupposes a certain degree of mental preparation and prior enculturation; defining those levels is itself a decision made by the schools that offer opportunities to share in complex discoveries.
2) it's a process of mental openness and opportunity. This point might seem counterintuitive: many are those who criticize formal education as being a constraining force on the mind; after all, to succeed within a school, we have to abide by its rules, agree with teachers, not allow our minds to wander, and reign in some of our creative expressions. But if schools were a major force in constraining thought, they'd be criticized as bulwarks of conservatism; instead, they're often considered palaces of wacky liberalism. Why? Because formal education teaches us ways to think and to be creative without being foolish. It frees us from the constraints that would otherwise be imposed upon our thinking by our immediate environments: the people and daily cultural beliefs and values that would limit inquisitiveness to the borders of whatever remained acceptable within the uneducated culture. Education rewards inquisitiveness, and at the same time provides us with tools of inquiry: the logic used for discoveries of thought, the scientific methods used for discoveries of knowledge, the artistic methods used for discoveries of aesthetics. By learning these tools we open our minds to the possibilities of making discoveries of our own that will be more likely to stand the tests of time and social scrutiny.
3) A formal education is also a credentializing force within society. It bestows symbols of completion, of honor, of power upon members of our society. These symbols ideally represent an achievement: becoming enculturated in human discovery to a certain level. It also makes it easier to inform others of this achievement. No doubt a person can achieve such enculturation without schools, but for the sake of efficiency in knowing whether this has happened, many societies rely on the credentials bestowed upon successful students. The credential also indicates that the person has what it takes to survive the schooling experience: the ability to follow institutional rules, some minimum of conscientiousness, some minimum of determination. This credentializing process has not been free from discrimination against specific types of people, and this is one factor distinguishing its ideal from its real practice.
There are some things an education might be thought to be, but isn't, or isn't entirely; for example:
a) An education isn't an IQ. Intelligence and education are closely related, but not the same thing. Education will boost a person's IQ score, but a person can have a high IQ and yet be barely educated. More likely is that a person will have a low IQ and be poorly educated. Intelligence (measured as IQ) is mostly a mental capability – it represents, in very simplified terms, the efficiency of the mind when tasked with manipulating and computing information; at the low end, for example, some minds are so inefficient that basic information like multisyllabic words quickly becomes too complex to compute. It generally takes a higher IQ to reach higher levels of formal education, but most of formal education can be achieved with IQs closer to the nation's average (albeit the lower your IQ, the more effort is needed to make the discoveries and enjoy the creativity that education offers, and an IQ of 100 - the national average - is low in relation to the demands of college).
b) An education isn't a detraction from common sense. Yes, there are highly educated people who cannot read a simple street map. Geography wasn't their major. But we have to put the anecdotes about the foolish behavior of some highly educated people aside; what counts as "common sense" is enculturation regarding some activities found in the culture. Formal education doesn't insulate students from the rest of culture, so they are as likely to learn whatever counts as common sense, as anyone else. However, some people feel better about themselves when they find faults in others, and so the accusation about shortcomings about those who are highly educated will no doubt remain with us.
c) An education isn't a sign of goodness. Neither is a person with much formal education better than those with less formal education simply because of it, nor does formal education expunge evil tendencies. Formal education attempts to promote values that most of us would consider good (e.g., empathy, non-violence, health, the betterment of humankind), but achieving success in formal education does not necessarily make you morally a better person.
d) An education isn't a sign of evil. Like I said, formal education attempts to promote values that most of us would consider good. It also promotes values and shares discoveries that some people think are evil, such as secularism, individualism, or simply, how to make a nuclear bomb. For some people, knowledge and thought is a threat – these represent a power that lead to change, and for some, change is unwanted.
e) An education isn't an exception from the real world. It's a part of the real world. Or imagine the real world without it. Most of the people who refer to the "real world" outside school mean that there is a more cruel environment outside the oft-forgiving setting of a school; after all, we live in an individualistic, capitalist society. Happily, those with more education tend to do quite well in that society.
f) An education isn't value-free. It's probably safe to say that most formal education in America is secular, democratic, liberal, humanist, and capitalist. Some formal education occurs in religious-based schools where Biblical morality is explicitly part of the curriculum, but the vast majority of schools in America are not formally tied to religious institutions, and public schools can not legally be so tied. Yet values permeate the education system: we promote the ideals of democratic societies; we value freedom, or liberty, so deeply that both personal choice (and the responsibility that comes with it) as well as profound acceptance of the superiority of a free society are promoted; we fancy ourselves a benevolent people, and this sense of caring is brought to the discoveries we seek, promote, and pass on; and we are also deeply capitalist, enjoying the fruits of monetary greed and the freedom to fail seeking it, which informs many of our assumptions about human behavior and drives many of the choices schools make about which topics they'll offer.
g) An education isn't expensive. Schooling is costly. Running a school is costly, and since the general public funds only a portion of higher education through taxes, the remaining costs (covering everything from the showers in the gym to the computers in the lab to the pavement for the parking lot to my measly salary as a teacher) have to be covered by those who use the school. But the education itself isn't so costly. Done without schools it would be quite cheap, but equally inefficient. And despite the price of the schooling, those who achieve higher education (and its credentials) receive far more benefits from society (for example, in income and social status) that it beats out any other possible long-term investment of the monetary cost (except, obviously, some very lucky and extremely rare business startups and gambling bets). Indeed, the best chance a young person has of becoming a millionaire in American society is to take what money he or she has and use it to get a good education.