OllyDbg is an x86 debugger that emphasizes binary code analysis, which is useful when source code is not available. It traces registers, recognizes procedures, API calls, switches, tables, constants and strings, as well as locates routines from object files and libraries. According to the program's help file, version 1.10 is the final 1.x release. Version 2.0 is in development and is being written from the ground up. The software is free of cost, but the shareware license requires users to register with the author.[2] The current version of OllyDbg cannot disassemble binaries compiled for 64 bit processors.
OllyDbg is often used for reverse engineering of programs. It is often used by crackers to crack software made by other developers. For cracking and reverse engineering, it is often the primary tool because of its ease of use and availability. It is also useful for programmers to ensure that their program is running as intended.
copy from{http://en.wikipedia.org/wiki/OllyDbg}
I would highly recommended download the Crackme and OllyDbg, and try this own your own first :P
watch video at
http://www.securitytube.net/Cracking-a-Crackme-with-OllyDbg-video.aspx
http://www.youtube.com/watch?v=dsqYKr61v48&feature=related
download at
http://www.ollydbg.de/
