There are three kinds of Open Beta. Usually a program will undergo two kinds of Open Beta, but there are many games that only do one or the other. The third type isn't so much a Beta as it is a label for a game they reserve the right to change.
There is the "application" Open Beta, where the company selects Betas from the applicants. This is primarily a bug testing beta, many companies would prefer people willing to give them feedback and report bugs for them. Otherwise they will want to get their demographics as wide as possible, both in terms of players and computer hardware.
Then there is the "load test" Open Beta. This usually occurs after the "application" beta. Anyone who signs up for this style of beta is let on so they can test how the server reacts under stress. These betas are also part of the "pre-order" reward scheme. They let everyone who pre-ordered on a day or two 'early' to seed things like the auction house and trading systems.
The third is the "unfinished" Open Beta. Anyone who signs up is allowed to play and the game opperates normally, they just never get rid of the Beta tag. This gives two advantages. It allows them to say the game is in progress when they make massive balance changes. The second, which I have never actually heard of happening, is it allows them to do a total server wipe when they offically go from beta to final. To the best of my knowledge 100% of all unfinshed betas are free-to-play games and no game in "unfinished" beta has ever gone to final.
Personally I think Guild Wars 2 will only have the "application" style beta. Being a retial program there obviously won't be the "unfinished" beta.
Arena.net is not accepting Beta applications at this time. Until then any application you might see related to Guild Wars 2 is a scam. When they finally do set-up the Beta you will be able to sign-up on the offical Guild Wars 2 site.