"What data is contained in a binary file?"

That depends upon the nature of the file!
If it's an image file, then it contains a binary representation of the image;
If it's an audio file, then it contains a binary representation of the sound waveform;
etc, etc

"Can I convert Ascii to bin?"

Of course, but the nature of the conversion would depend upon how "ASCII" has been used to represent the underlying "binary" information; eg Intel Hex and Motorola S-Records are both ways to represent the binary contents of a memory device in an ASCII format.

Keil's Utility download page has some conversion tools:
http://www.keil.com/download/utility.asp