Combinations of pokemon supporting each other is what keep a team alive. Defensive combos for Stall teams, and Offensive combos for Offense teams.
Offensive combos usually consist of two pokemon, one with a setup move such as swords dance or nasty plot, and another with a choice item, or a priority move to get rid of your setup pokemon's counters. These pokemon are usually balanced so that one of them is physically offensive and the other is specially offensive.
A classic example of the offensive duo is Dragon Dance life orb Salamence, and scarf Heatran.
There are two types of defensive combo. the first is normally a mix of a physical wall and a special wall. Both of them usually carry a reliable healing move, and they normally have two status moves between them. The most well-known of these combos is SkarmBliss. The second type is that of a tank, a special wall, and a physical wall. They are most effective when they only carry one status move between them, but it is recommended that one of them be a ResTalker.
An example of this is ResTalk Suicune, Bronzong and Porygon2
please feel free to comment with some new combos, or feel free to ask any of the previous combos.