Back in Maven1.0, plugins and normal artifacts were in different repositories (as far as I know). In Maven2+ they are stored together in a repository. But there are many different repositories out there from many different companies and projects so you have to pick the repositories you need.
A plugin is a tool you use at the execution of your maven build. For example, the compiler plugin compiles your Java code into Bytecode.
A normal dependency are artifacts you use in your application like libraries.
Both plugins and artifacts can have transitive dependencies. That means they have dependencies themselves.