2345 is an old joke regarding what an Elemental Shaman pushes to win - as I understand it.

cleave in a comp name typically means focusing one target down very quickly, and originally applied to heavy melee teams (warrior/rogue/druid). 2345 is exactly what peeeriest said. Comps like TSG come from the name of the team who first played it. wiz cleave is basically the same thing as any other cleave team except with huge magic damage instead of melee.

The name came from a very highly ranked Ele Shaman in BC who siad he bound his 2, 3, 4, 5 keys to Bloodlust, Elemental Mastery, Nature's Swiftness, and Chain Lightning.

In the first 3 seconds of the match he would buff his whole team and unleash crazy high burst on the other team and all he had to do was roll his fingers on the numbers. After that, he joked, his job was done and he just ran around until his cooldowns came back up again.

There was also a 2346 setup where you replaced the mage with warlock, i asked once why the 6 and my friend told me because warlocks need 1 extra button. Never new if that was true but makes me laugh

The names for various arena comps have become a bit of a joke, and arena junkies now has a thread dedicated to collecting all the different 'cleave' names for comps (http://www.arenajunkies.com/showthread.php?t=135411). Some make sense (beastcleave for enh shaman, BM hunter, resto druid or other healer), others, such as African turtle cleave, are a little bit harder to understand.