Another important use for at and in, as well!
Defines a place that the object of a sentence occupies. It can also specify a time that something is happening.
Example: I have to work at 4 o'clock. (Tengo que trabajar las cuatro.)
Defines a noun being 'inside' another noun (as Jay pointed out - We are in London, for example). Can also be used to express time as the amount of time until something happens.
I go to work in five minutes. (Voy trabajar en cinco minutos.)