If you want to know why one would need to Root the phone, then please google the same. Listed below are the instructions for rooting your phone and works like a charm on my phone.
Happy Hacking :)
1. Download z4root.1.3.0.apk from http://www.filefront.com/17562655/z4root.1.3.0.apk/ (If this link is dead you can alo try google, if you can't find it, leave a comment with an email address.)
2. Connect your phone to your PC via USB, turn on USB Storage and paste this file in the Downloads folder on the Removable Drive of your phone (for uber n00bs, advanced users may feel free to shove it anywhere)
3. Now after the file has been copied, Turn off USB Storage from your phone, wait for SD card to be ready and then unplug the USB Cable from the phone.
4. Press the central launcher button and scroll to OI File Manager (or any other file manager you have) and open the downloads folder.
5. Touch the z4root.1.3.0.apk file and install it.
6. The screen will stop and will go white for sometime (around 30 seconds) and then reboot
7. Once rebooted, touch the central app launcher button and scroll down to the downloaded apps and click on z4root icon.
8. If it shows the option to Re-root or Un-root, congratulations!
9. If it doesn't, try googling and getting a fix (Post comments here and I'll update you with it ASAP)
Hope you found this article helpful in rooting your Samsung Galaxy 551.