Another important use for at and in, as well! At: 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.) In: 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.)

In general, we use: at for a POINT (ex: at the bus stop ) in for an ENCLOSED SPACE (in London) on for a SURFACE (on the wall) read more here : http://www.really-learn-english.com/at-in-on-prepositions-of-place.html :)