Yes, ODK Cloud, is official managed hosting of Central on ODK's infrastructure. It's a paid service that guarantees hassle-free levels of speed, reliability, and security for your project.
There's no limit to the number of form submissions per month if you are self-hosting Central. On ODK Cloud, there is a generous limit on submissions per month that varies based on the subscription you choose. http://getodk.org/#pricing has more information.