A verb’s aspect is closely related to a verb’s tense.

tense = when the action took place
aspect = what kind of action took place

GGBB teaches three different kinds of aspect:

  1. internal – (the Greek present and imperfect tense)
  2. external – (the Greek aorist tense)
  3. perfect – (the Greek perfect tense)

External refers to an action as a whole. Some would call it “snapshot” action.

I ran the race.
We built the house.

Internal refers to action as ongoing, continuous, repeated, or habitual. BBG call this ” continuous aspect”.

I am running the race.
We were building the house.

Perfect aspect refers to any verb tense that speaks of the present consequences of past action. Wallace calls this perfective-stative. See GGBB p. 499-503.

I have run the race.
We have built the house.

A long list of aspects rendered into English can be seen here.  Hebrew verbs also have aspect. See here.  This post is also interesting.
Optional, more advanced information:
Wallace distinguishes between aspect and Aktionsart:

“Aspect is the basic meaning of the tense, unaffected by considerations in a given utterance, while Aktionsart is the meaning of the tense as used by an author in a particular utterance, affected as it were by other features of the language.” (source).