All About "About"
It probably will come as a surprise to learn that there are several ways that about is used in English and each has a different solution in Spanish -- and often an alternative or two.
By far the most common translation is sobre (also used for above,so be careful and learn more about this word! In its meaning as about, sobre can also be substituted with acerca de. They are used when in English, the word about means concerning or having to do with or dealing with a particular idea or topic.
Es una clase sobre/acerca de la química orgánica (It's a class about/dealing with organic chemistry).
Another common solution is the very common preposition de, which corresponds to about in the sense of concerning -- often when a person is the topic of "concern":
¿Has oído hablar de esa chica? (Have you heard about that girl? -- in the sense of "what is said of her").
Yo sé mucho sobre los francmasones (I know a lot about the Freemasons).
The use of de isn't limited to knowing about people. It can refer to a whole body of information:
Por favor, quiero saber más de todo eso (Please, I'd like to know more about all that).
Sometimes, English uses about in the sense of conveying an approximation, as when speaking of numbers or clock time. When this is the case, the phrase a eso de is used, in the sense of around, or 'round about:
Mi amigo se despertó a eso de las siete (My friend woke up about/around seven).
Similarly, the word sobre can be used colloquially, when expressing such approximations:
Por favor, pase a buscarme sobre las cinco (Please come by for me 'round five).
When rounding figures or other statistics, cerca de or unos (and unas) are also very common:
Hay cerca de cien personas en el tren (There are about a hundred people on the train).
Tengo unas docenas de alfombras pérsicas (I have a few dozen Persian rugs).
Still another solution is to use más o menos is another way of approximating, and is not limited to numbers, but also qualities:
Hay más o menos cincuenta dólares en la caja (There are more or less fifty bucks in the till).
Juan es más o menos alto (John is kind of tall).
Finally, if you want to know what something is about, you can also use the verb tratarse de:
¿De qué se trata este libro? (What is this book about?)