The two exceptions to this are the .AU and .AS ccTLDs, which are automatically registered for two years per requirements from those registries.
All domains are set to renew automatically, so you will be billed for the domain renewal once each year (or once every two years for .AU or .AS domains) unless you intentionally cancel the renewal. We will notify you well in advance of the domain renewal.
Different TLDs (domain extensions like .COM, .CA, .PHOTOS, etc.) are priced differently for registration, depending on pricing set by their respective registries. See more information on domain pricing here.
TLDs sometimes go on sale, so you might have registered the domain at a discounted price. Renewals are always billed at the regular price, however. So a domain may seem to have gone up in price when renewal is billed, but it's just that the sale does not apply to renewal. Our pricing page shows both sale and regular/renewal prices.
These articles may also be of interest:
If you've been wondering about increasing domain prices
Why domain prices can vary among registrars