Alright, first up, the Setting and Background.
The basis for Avatar in is a world with 4 groups of people. The Water Tribes, the Earth Kingdoms, the Air Nomads, and finally the Fire Nation. Each group also has people who are called "benders". These benders have the ability to manipulate the element to which they people are aligned. IE Fire Benders can generate and control flame, Water benders handle water, Earth Benders deal with Earth, and Air Benders handle, guess what?
Thats right! AIR! :P
Anyway, in addition to these benders (the power of which vary from person to person) there is a single person called The Avatar. This person, who is born in cycled between tribes, has the ability to learn and control all 4 elements. The cycle is Water, Earth, Fire, Air. Each time an Avatar dies, he/she is reincarnated into the next nation in the cycle. The current Avatar is Aang, the last living Airbender. Hence the name of the show! :)
Basically the show is divided into seasons, and each season thus far (there have been 2) have dealt with Aang and his friends on their journey to defeat the Fire Lord. You see, Aang was born 100 years prior to the start of the series. When he found out he was the Avatar, and that he would be seperated from his Guardian, a monk named Gyatso, he ran away from home on his flying Bison named Appa (who rules btw and is fluffy!). They got caught in a huge storm out near the South Pole, and went down in the ocean. Aang then went into self defense mode, called the Avatar State, and froze himself and Appa in a giant iceberg. 100 years late, and Sokka and Katara of the Southern Water Tribe find him. That is what happens in episode one. During the 100 years he was gone, everyone thought that the Avatar was dead and gone, and the Fire Nation attacked the other countries in a bid to take over the world.
Aangs mission? Master Water Bending, Earth Bending, and then Fire Bending (in that order) and then defeat Fire Lord Ozai.
Reasons I like the Show!
There are no stupid villains for one (Jess pointed this out). Even Admiral Chao (Whose, as Jess put it, Ego has its own Zip Code) isnt stupid.
The plot is actually very interesting. The fact that they dont try to force any of those goofy anime expressions is another, and I have yet to find the story or adventures flat. I love every character that has popped up (Even that ass Zuko :P).
So if you are looking for something that will keep you entertained? Avatar, the Last Airbender is something you want to get into :D