SQL is the basic ANSI standard for accessing data in a relational database. When you see “MSSQL” it is referring to Microsoft SQL Server, which is the entire database architecture and not a language. T-SQL is the proprietary form of SQL used by Microsoft SQL Server. It includes special functions like cast, convert, date(), etc. that are not part of the ANSI standard.
You will also see things like plSQL, which is Oracle’s version of SQL, and there are others as well (mySQL has its own version, for example, and Microsoft Access uses Jet SQL.)
It is important to note the the ANSI standard for SQL has different releases (for example, 92 or 99, representing the year it was released.). Different database engines will advertise themselves as “mostly ANSI-92” compliant or “fully ANSI-99” compliant, etc, and any exceptions will usually be documented.
So although “SQL is SQL”, every engine uses its own “flavor” of it, and you do have to do a little reading on the particular platform before you just dive in.
A further note – the SQL extensions, like T-SQL, are generally considered full-fledged programming languages, complete with looping, if/then, case statements, etc. SQL itself is limited to simply querying and updating data and is not considered a true programming language.
Wikipedia has a decent article for an overview here:
http://en.wikipedia.org/wiki/SQL