Domain Name Registration

Can someone explain to me why it is that domain names cost either...
< $10 a year
or
$35 a year

What's the difference?

If the OP was refering to price differences between different domain types as >>5 suggests, the top level domains that are usually the cheapest of the lot is because most of the namespace and common word choices are taken. The newer domain types are typically more expensive as they are considered "premium" to a certain degree and have a considerable larger domain namespace that remains untaken.