I recommend that you see the "How TestComplete Recognizes Processes, Windows and Controls" help topic. In addition to that, here is some information for new TestComplete users:
First of all, I recommend that you see the "Getting Started" section of TestComplete's Help system. A printable version of this section (Getting Started With TestComplete 6.doc) is located in the "<TestComplete 6>\Help" folder.
The "Working With Applications' Objects and Controls" help topic can also be useful for you.