It can be as specific or as generalized as you want it to be, and it's not set in stone once you've done it. It can be updated as often as you want.

When you contact a lawyer about having a will done, they will ask you to itemize your assets. They'll also ask you how you want those assets divided among your heirs, and whether you want specific people to have specific items. The lawyer will then put it all into the proper legal form for your state.

Once it's done, make sure that your relatives (particularly the one you designate to be your executor) have copies of it, and that the original is in a known safe place, since that original has to be given to the probate court after your death so that your wishes can be carried out as you specified.