The quick and dirty version:
Linguists describe what verbs do by various methods, but one way is to discuss tense, aspect, and mood.
Tense = time, as in you have all of three choices here: past, present, and future.
Aspect is how the time is tweaked, how it is described. There are various ones here: punctual (with a definitive beginning and end; usually either the preterite or the aorist), durative (happens over a period of time, and often doesn't have a definitive beginning and end; imperfect, progressive), perfective (usually relates to two different tenses; perfect and sometimes the aorist), among others. Indo-European languages, as I recall, generally have a fair amount of aspect in the languages, but are no where near the aspectual level as, say, the Austronesian languages.
Mood is kinda just that: what's the perspective of the speaker. Indicative is just a statement about the world, usually without emoting anything. Subjunctive, and its cousin the optative, impart doubt, emotion, will, and other 'subjective' points of view; optative is the will of whatever Supreme Force the culture believes in, and often includes supernatural phenomena. Imperative is usually a direct command. For many languages, this is the limit...but there are other options, I believe.
So, that being said:
Present: tense
Perfect: usually a puntuctual or perfective verbal form, depending on the language. In Latin, it tends to be more punctual, but the modern Romance languages have compound perfect tenses which are perfective in aspect.
Imperfect: a past tense form which is durative in aspect.
Hope that helps!