A basic figure is essentially the figure as it looks in the show (or as close as possible) as how many would recognize it. In some instances basic figures are released only once or twice in a line, such as in DC Universe Classics or Masters of the Universe Classics (where the limited releases of figures give way to fan demand of the return of figures). At times, however, as in the case of Justice League Unlimited (toyline) they also become staple figures, which appear multiple times in a line mostly to 'carry' other figures that are packed in with it.
To be very technical, it's possible for staple figures not to be basic figures as they are not screen accurate. Examples of this are Batman 2 ver 7, and Green Lantern 2 ver 7, both of which come close to being show-accurate but are technically redecoes.