Players changing forms

I'm really pretty new to guild wars and I've seen so many people running around Ascalon (post-searing) with all kinds of different forms. For example, I've seen a player who looked like a char. Another who looked like a big boney tiger, and still another who looked like a spiffy ice elemental.
My question is how are these people doing this? I've click on them and they seem to be of varying classes. If anyone can help me figure out how to change like them I would be really grateful.

Tonics

wiki.guildwars.com is an invaluable source of information for beggining players as well as veteran players. can answer many questions u may have ;-)

As long as you know where to look :P

Their new player guides are pretty good to get you started aswell.

Oh, thank you very much. My problem was I didn't know what I was looking for. I had googled for a few days but all I came up with was a few Dervish skills that have something to do with forms.
Just one more question if anyone can answer, how would one go about getting one of these tonics? Are they sold from an npc or are the cash shop only?

U can buy em from ppl craft em from events or drops ,zaishen chest.

on the tonics page, listed above, click on the link in the acquisition column and it should tell you. you can also purchase them from other players.