Alliance
An alliance is a group of players that works together primarily in attacking and defending.
In the current Planetarion setup, Alliances are part of the game. Since Round 10, joining an alliance in-game has allowed all the planets within that alliance to defend each other with -1 Travel Time. Players can apply to join an alliance from the alliances page in-game. Most alliances also have an IRC channel, and it usually best to enquire about joining there before applying in-game.
The Universe page has a section on alliances which shows the ranking of the top 100 alliances in the game. This data is compiled by addding up all of the planets signed up to the alliance in-game.
Joining an alliance is considered a vital part of the game, both for full enjoyment and for chances of success. One's alliance is a source of defense and attacks, as well as a place to socialize.
See Also
This page needs some more information about the ingame tag system along with perhaps some history of alliances.
