In professional wrestling, a "tag team" is a unit
comprised of two (or more) wrestlers who compete together in tag
team matches against other teams.
In normal tag team matches, only one wrestler from each team
starts out in the ring. In order to switch off to a different
member of the team, the wrestler in the ring has to
"tag" in the wrestler on the outside by slapping their
hand. In the good old days, there was a short loop of rope
attached to the ringposts--the wrestler on the outside had to be
holding on to this rope while the tag was being made for it to be
legal. Nowadays, the rope is usually absent and the wrestlers
just have to be in the "vicinity" of their corner.
After a tag, it is legal for the wrestler who has just been
tagged out to doubleteam his opponent with his partner for five
seconds; he must subsequently vacate the ring. It's also legal
for a wrestler on the outside to enter the ring to break up a
pinfall attempt, as long as they immediately leave the ring
afterwards. Tag Team matches end much the same way as regular
matches--by pinfall, submission, disqualification, or
countout.
There are numerous variations on the tag team match:
Elimination Match: When one member of the tag
team has been pinned, submitted, or otherwise disqualified, the
match isn't over--that wrestler is eliminated from the match, but
his partner(s) continue without him. The match only ends when all
the participants of one team have been eliminated from the match.
A Survivor Series match is a special type of Elimination Match
in which there are four participants on each team.
Texas Tornado Match: The tag rules are
omitted; all the wrestlers from both teams are in the ring at the
same time.
Handicap Match: One team is bigger than the
other(s).
And, of course, most variations on regular matches can be applied to tag team matches--ladder matches, table matches, strap matches, etc. Hell, you could have a Kennel From Hell tag team match if you really wanted to...