In the last Busiek Defender's series, the team was Dr. Strange, Namor, Hulk, Silver Surfer, Nighthawk, Hellcat and Valkyrie. That's considered the classic team more or less, with Strange, Namor, Hulk and Surfer being the big 4. There have been numerous other Defenders throughout the course of the team's tenure.
As to who wil be in the new mini-series, I think it's just going to be the Big 4, with maybe apperances by others.
http://jerome.galica.free.fr/marvel/defenders/defenders.htm
Micro-heroes are cool.
ScarletSpider is correct.
first there was Titan's Three which was namor, hulk and surfer. then the defenders formed with namor hulk and strange. then surfer joined. then valkyrie and black night joined. then hawkeye joined. then black night left. then hawkeye left. and it all just gets pretty complicated from there. . .
however the line-up for the second volume is
Strange
Hulk
Namor
Surfer
Valkyrie
Nighthawk
Hellcat