The term aspect is unfamiliar to most English-speaking students, despite the fact that English verbs have aspect as well as tense. For example, the forms took and was taking are both in the past tense; the difference between the two is one of aspect. (The difference between tense and aspect is also discussed in Verbs: Aspect.)
English and Russian differ considerably as to verbal aspect. English has a special ending for on-going action, the -ing form as in was taking. Russian, on the other hand, has particular verbs, Perfective verbs, that specify the end-point of an action, such as взїл, which means 'took (and it is gone)'.
The forms взїл (Perfective past) and брђл (Imperfective past) both mean 'took'. Use взїл when you want to specify that the action has an end point. For example, Ћн взїл кнќгу specifies that the book is gone — the end point of the action of 'taking' is that the object is no longer there. If, on the other hand, the book has been returned, then you can't use взїл; in this situation it would not be appropriate to specify the end point of 'taking', because the object is back in the owner's possession and therefore you must use брђл: Ћн брђл кнќгу.
Both of these sentences can be translated 'He took the book', but they mean different things. Ћн взїл кнќгу implies Кнќги нљт 'The book isn't here'. Ћн брђл кнќгу does not have that implication; it has any other kind of past tense meaning: 'took (and brought back); (often) took; was taking', etc.