Using libraries written in a language other than the working language

I asked a senior developer at my company how he uses a software library written in a language other than his working language — he is mainly working in C#. He named three possible techniques to do so: 1. COM (Component Object Model), 2. Reflection, 3. Wrapper libraries. I knew a little bit about Wrapper libraries, and about a fourth idea of Foreign Function Interface; but I had never heard about using COM to use foreign libraries and have no idea of Reflection. Can you please give some basic idea of the first two, point to some resources where I can learn more about these techniques? Some reference to any good resource for writing wrappers would also be beneficial.