difference between sqlexpress and mssql 2008

I have two SQL Server services in Sql Server Configuration Manager: SQLEXPRESS and MSSQLSERVER. I have no idea what the differences are. I think that SQLEXPRESS is the free version, but I don't know how I got it and I can't remove it either because it doesn't show up in remove programs.

But here's where it gets weird: I installed SQL Server Enterprise, and during installation I speci开发者_运维问答fied a local user (SQLServices) to be used for all SQL Server services. Okay, so this worked for SQL Server Analysis Services (MSSQLSERVER) and SQL Server Integration Services10.0 (MSSQLSERVER), they are running under this user. But SQL Server (MSSQLSERVER) does NOT run and gives an error that it can't connect/time out etc., and SQL Server (SQLEXPRESS) runs, but under NT AUTHORITY\NETWORK SERVICE. I stopped this, and tried to run the SQL Server (MSSQLSERVER), but it keeps timing out on me. What's going on?

Here's a comparison of the different versions:

SQL Server Features