Dribbble is establisted by Rich Thornett and Dan Cederholm at 2009. It allows designers to share what project they are working on, and allow other designers to comment on their current projects. So it creates a ideas sharing area. Dribbble is a Twitter style website and it's asking "what are you working on?" instead of "what are you doing?" to it's users.
There are 2 types of membership for the site. If you are looking for a designer for your projects, or your purpose of membership is follow new beautiful designs, you should sign up as "Prospect" and make a designer to invite you as a "Player".
Membership is briefly based on an invitation system. Membership is totally free. But if you want to upload somethings on website, you should be invited by a "Player". The reason of this is, make the "Player" responsible of behaviours of who he/she invited. Whoever makes an attitude, it's also effects the "Player" who sent invitation.
Dribbble is ideal for designers who wants to show his/her talent to the others. Site gives you a chance for inspire to others, or also get inspired by the other designers. It gives you a chance to upload 24 designs on the 1st day of each month. This number is static. You can transfer you quota to next month. Example, if you uploaded only 10 and left 14 at April, you can upload +14 more at May.
If you are a "Player", you can share your designs, get comments from other designers, you can make your designes "Like", or even can share your projects source codes. (Sharing files is for pro accounts only)
You have seen a design on the website and you believe you can imrpove it. You want to improve it and then share it to the other designer. Here "Rebound" feature comes up. Rebound makes you to answer a design with another design. So you can answer someone with a perceptible design of your own instead of commenting by words. It's also valid your your own designs. You can show your followers to how forth your design is improved and also your followers can share their ideas on your projects.
Another feature is "Bucket". You can arrange designs which you like by grouping in categories and makes you access them faster. Your followers can see you to what design you "Bucket" into what category. So other designers can reach same "Buckets" and categories you are using.
Last, i want to write about "Jobs" page in the site. You can reach out employment ads. You can read what companies looking designers in what criterias. You can also apply for jobs listed. You will see a lots of ads "designers wanted"
You can ask anything you want about Dribbble by comment box below. I will try to answer as much as i can. You also can follow me at dribble by : http://dribbble.com/dailydesignnotes