The Tenkaichi Budoukai is a martial arts tournament that is held once every three years on the 7th of May. However, there are always exceptions. For example:
11 years passed between the 23rd and 24th Tenkaichi Budoukais because the arena was being rebuilt.
7 years passed between the 24th and 25th Tenkaichi Budoukais because of the Cell Games.
4 years passed between the 25th and 26th Tenkaichi Budoukais because of the fight with Majin Buu.

The best fighters from around the world always gather for the tournament because the winner is declared the world champion and receives a luxarious prize. Now, what does Tenkaichi Budoukai mean? First, let's look at the composition of the words:
Ten (Heaven or Sky) combined with Ka (Under or Beneath) basically creates "Under the Heavens". And what's under the heavens? The Earth of course. Therefore Tenka gives us "World".
Ichi basically means "First" or "Best". When combined with Tenka, we get Tenkaichi- "The World's Best".
Bu (Martial Arts) when combined with Dou (way of) basically gives you "Martial Arts." Now, you're probably thinking, where did the "way of" go? Well, because "Martial Arts" means "way of fighting", "way of" is already accounted for in "Martial Arts".
When Kai comes after Budou, it basically means "Martial Arts Meeting". Well, martial arts fighters don't just meet, that battle right? And what do they hold the fighting in? A tournament of course. Therefore, Budoukai gives us "Martial Arts Tournament". Now just combine the two parts, and you get "The World's Best Martial Arts Tournament".

There are also tournament rules. You are not allowed to:
Poke your opponent in the eye.
Give a low blow to your opponent.

You are disqualified if you:
Use any foreign object or weapon.
leave while the tournament is still in progress.
kill your opponent.

You lose if you:
are knocked unconscious.
are down for a count of 10.
are knocked out of the ring
say "I give up".

Previous champions and runners up:
The 21st Tenkaichi Budoukai:
Champion: Master Roshi
Runner-up: Son Goku

The 22nd Tenkaichi Budoukai:
Champion: Tien
Runner-up: Son Goku

The 23rd Tenkaichi Budoukai:
Champion: Son Gokou
Runner-up: Piccolo

The 24th Tenkaichi Budoukai:
Champion: Mr. Satan
Runner-up: Unknown

The 25th Tenkaichi Budoukai:
Adult Division Champion: Mr. Satan
Adult Division Runner-up: Android #18
Children Division Champion: Chibi Trunks
Children Division Runner-up: Son Goten

NOTE:
Mr. Satan is the champion and Buu is the runner up in all of the tournaments to follow. This is because Mr. Satan bribes Buu with candy let him win the tournament. Buu defeats all of the other fighters and then throws the final match to Mr. Satan.