The Ultimate Boot CD is a single CD with many diagnostics compiled on it. There are two primary uses:
Hard Drive diagnostics - every manufacturer has a Diagnostic tool they use to determine if they will RMA a drive - also these can sometimes test codes generic tools cannot, so always verify a failure condition from another tool with the proper manufacturers tool if in warrenty to hopefully RMA. Always run the "Advanced" or "Long" test, the fast test will only catch SMART failures which should be noted by the BIOS anyway. Remember that SpinRite6 will often detect a declining drive before the Manufacturer test will, so it is recommended to start with that tool.
Memtest86+ - Pretty much the standard and most reliable software memory tester. Will usually detect a bad RAM chip.