In linguistics, the grammatical aspect of a verb defines the temporal flow (or lack thereof) in the described event or state. For example, in English the difference between I swim and I am swimming is a difference of aspect.
Aspect, as discussed here, is a formal property of a language. Some languages distinguish a large number of formal aspects (see the list below), while others distinguish none at all. Even languages that do not mark aspect formally, however, can convey such distinctions, if important, by the use of adverbs, phrases, serial verb constructions or other means.
Present Simple (not progressive/continuous, not perfect; simple): "I eat"
Present Progressive (progressive, not perfect): "I am eating"
Present Perfect (not progressive, perfect): "I have eaten"
Present Perfect Progressive (progressive, perfect): "I have been eating"
...and for the past tense:
Past Simple (not progressive/continuous, not perfect; simple): "I ate"
Past Progressive (progressive, not perfect): "I was eating"
Past Perfect (not progressive, perfect): "I had eaten"
Past Perfect Progressive (progressive, perfect): "I had been eating"
Another aspect that does survive in English, but that is no longer productive, is the frequentative, which conveys the sense of continuously repeated action; while prominent in Latin, it is omitted from most discussions of English grammar, as it suggests itself only by Scandinavian suffixes no longer heard independently from the words to which they're affixed (e.g., "blabber" for "blab", "chatter" for chat", "dribble" for "drip", "crackle" for "crack", etc.).
