COM is primarily used in-process, it exposes an ABI that allows multiple languages to operate on a common object model within the same process. That's why you can import COM controls into a .Net application by referencing the DLL in your project and work with them as if they were native CLR objects - or Python, Ruby, Perl, etc. for that matter.

It lets you use libraries written in numerous supported languages with a richer, more object oriented interface than a C ABI. You can use the same underlying interface to call out of process or distributed objects.

COM is Common Object Model. What you mean is OLE. In process COM calls are basically direct DLL calls and therefore very fast.