Basically, it's pay what you can afford. There are tool tips on the membership form, but they might not be visible in all browsers.
Unwaged: For students, beneficiaries, children, and others on low incomes. Basically, select this if you genuinely can't afford the Standard rate.
Waged: The standard rate; if in doubt, select this.
Solidarity: For anyone with a bit of extra cash to spare who'd like to help make the convention better for everyone.