Combo

A combo (not to be confused with a string) is a sequence of attacks that lead into another without the opponent leaving hitstun. Combos are not necessarily guaranteed however - many factors play into their effectiveness, such as DI, SDI, teching, damage, weight, and timing of the attacks.

History
Combos exist in every Super Smash Bros. game, and are a crucial technique for both mid-level and high-level play, and are considered as a large factor when deciding a given character's placement on tier lists.

In Super Smash Flash 2
Creative combos were not possible in the Super Smash Flash series until demo v0.5a, when the engine's knockback physics were retooled specifically to allow them, and characters were not coded by Developers with combos in mind until v0.7 (bar v0.5a's Sonic). Prior to v0.5a, strings were very prominent and were actually referred to as combos by most of the community. All characters since the engine rehaul have been designed to combo using different attacks, but some are designed to do so better or worse than others.