The aspect of a verb phrase indicates whether the action described by the verb is still happening or is complete. The difference between the perfect and the present continuous aspects is what makes the difference between sentences such as "I have baked a cake" (the cake is ready to eat) and "I am baking a cake" (I am up to my elbows in flour).
Like tense and mood, aspect communicates shades of meaning. In English, the meaning communicated by aspect is mostly to do with the completeness, or otherwise, of the action. The perfect is conveyed by using the auxiliary 'to have' and the -ed participle; the continuous is conveyed by the auxiliary 'to be' and the -ing participle, again potentially with other verbs. The simple aspect in the present tense is conveyed by the base form of the verb, with an added '-s' in the 3rd person singularand in the past tense by the past tense form.
English verbs can have three aspects.
- Perfect comes from a Latin word meaning 'finished'; and although its meaning in grammar is much more complicated than this simple fact, it makes it easier for some students to use the word correctly. "I have made the supper" implies "and it's ready now", or "it's finished [~ perfect]". This is the present perfect; contrast it with the simple past "I made the supper", which has the more general meaning of 'some time in the past'.
- The perfect aspect can also be used to describe actions in the past: "I had made the supper". This feels like an unfinished sentence, because we are expecting an action to come after the supper was made: "I had made the supper when Uncle Horatio burst in." (Somewhat confusingly, "I have made the supper" is called the present perfect; "I had made the supper" is the past perfect, although both take place in the past. The tense of the auxiliary 'to have' is the clue: 'I have' is present tense.) The past perfect was traditionally known as the pluperfect, from the Latin plusquamperfect, via French plus-que-parfait.
- The continuous aspect, also called the progressive aspect, expresses a feeling of something continuing to happen, or in progress. This uses the auxiliary 'to be' to construct verb phrases, as in "I am going" for the present continuous, as opposed to "I go" (the simple present), or "I was singing" (the past continuous) as opposed to "I sang" (the simple past). If I say 'I am singing', I am singing right now. However, the precise meaning of the continuous aspect can be modified by the rest of the phrase: "I am singing in the concert on Tuesday", or "I was singing in a band the year I met your mother."
- If a verb is clearly neither progressive nor perfect, we call it simple - in the simple aspect. These are the tenses formed by inflection: "I sang", "I baked a cake".
AWE has a table demonstrating this which you may want to look at.