最近碰到这个问题，搜索了一下
In-proc - Com object is present within application process
Out-Proc - Com object has it's own process area
In-proc - E.g. Activex Dll
Out-Proc - E.g. Activex exe
In-proc - Faster
Out-Proc - Comparitively slower
In-proc - Useful for local com
Out-Proc - Useful for remote com
P.S:
http://wiki.answers.com/Q/What_is_the_difference_between_in-proc_and_out-of-proc