A combo, as the name implies, is a series of attacks that hit in succession before the opponent has time to recover from any of the hits.
Combos usually take advantage of cancels, links, and juggles. In many fighting games, including M.U.G.E.N, a successful combo will be accompanied by a visual indicator showing the number of hits in said combo.
Any combo where all hits of the combo are blocked are known as Block Strings. It should be noted that due to differences in hitstun and blockstun in many attacks, not all valid combos are valid blockstrings, as the opponent may be able to recover faster from a hit if it were blocked than if it were not blocked, allowing them to interrupt the blockstring.
However, for some characters, there exist infinite combos, sometimes called loops, which repeat or extend a portion of the combo infinitely until time runs out or the opponent loses all health. These are typically found in characters with improperly coded moves, excessive hit/block stun, or an ineffective juggling system. While infinites do exist in commercial games such as Marvel vs Capcom, most authors make a conscious effort to remove these infinites in order to maintain balance.