The clipboard on the iPad is a virtual thing - you can't access it as you're probably accustomed to on a laptop or desktop computer.
Once you copy something on the iPad, you can go to any app and then press/hold your finger in a text entry box and, when you get a little pop-up button, select "paste." Note that it has to be a text entry box or area - as the copy/paste function won't work if you copy a picture and then try to add it directly to the Photo app; you can, however, copy a photo and paste it into the body of an e-mail message...
And, it's a one-time deal (unless you're jail broken) in that every time you copy something, you're replacing whatever it was you copied the last time. In this aspect, it's pretty much the same as a lap/desk top computer.
Hope this clarifies.
Marilyn