Apple Users

you need to first figure out whether your hardware is supported for the release you want

Suppose, for example, you want to move up to Snow Leopard and it's compatible with your hardware. You may have several options:

(1) Buy a copy of Leopard. Buy a copy of Snow Leopard. Update Tiger to Leopard. Then update Leopard to Snow Leopard.
(2) Buy a copy of Snow Leopard. Try to update Tiger directly to Snow Leopard. Cross your fingers and hope it works.
(3) Make back up copies of everything you want. Do a clean install of Snow Leopard. Then reinstall your other software and move your old your files back onto the machine

(3) is probably preferable, in terms of reliability, but it's likely to be a hassle. (1) may be a more reliable option than (2) but it will also involve buying a copy of Leopard as well as a copy of Snow Leopard

Upgrades can bring unpleasant surprises, too. In Lion, Apple stopped supporting my Apple dialup modum