Shared MySQL - The databases are hosted on a MySQL service that is shared amongst other customers as well. While on a shared hosting environment, the resources of the MySQL server and service are not guaranteed to one user, and thus there could be conflicts that occur between customers. We do our best to keep these hosting environments optimized. A typical MySQL service will allow up to 300 connections at one time, that total of connections being shared with other customers.
MySQL VPS - The databases are hosted on a MySQL service isolated from other customers. RAM is guaranteed for the customer. The customer will not share the 300 connection limit for their MySQL service, it's all theirs. MySQL VPS is optimized with database query caching which is not in place on a shared MySQL platform.