Russian Verbs: Aspects
In addition to three tenses - past, present, and the future - Russian language has the concept of aspects that can be summarized as such:
- Imperfective - ongoing, repeated, habitual, or reversed action
- Perfective - completed actions
Many of the verbs appear in both forms, sometimes there is a preposition in front to mark the perfective case (Делать imperfective "to make", Сделать perfective "to make"), sometimes the verbs are completely different (e.g. Говорить imperfective, Сказать perfective)
One additional tidbit is that negative perfective doesn't just mean something didn't happen, it means, something failed to happen that was supposed to happen.
That is the executive summary, for more, see for instance Russian Lessons on Aspects.