This excerpt from Tech Republic explains it quite well (emphasis mine):
The Secure Empty Trash command prompts Mac OS X to perform a
seven-pass* erasure of the file Instead of just removing a directory
entry to the file or files in question, Mac OS X's Secure Empty Trash
command initiates a series of seven different passes in which random
information is written to the hard disk sectors previously occupied by
that file. In fact, Apple states that this secure erasure meets United
States Department of Defense security standards.
*This is incorrect. It's a 1X pass. See edit.
In comparison, "Empty trash" does not write over the 'deleted' data at all - it just marks the blocks where this data is stored on your HDD as available for use. It does not erase the actual data you discard.
Writing over a disk even one time takes a while. Simply telling the system "these blocks are now available to be written over" takes no time as well.
You should use "Secure Empty Trash" if you are deleting sensitive information and you want to make sure it won't be recovered in the future.
That Tech Republic article is incorrect. As Andrew Medico correctly pointed out in his answer, securely emptying the trash performs a 1X pass. From Apple's Training Manual:
There are varying levels of security offered depending on the number of passes and whether each path uses a specific data pattern or random data. Disk Utility in OS X offers multiple sanitization options for an entire volume or free space. A seven-pass erase option is available that meets U.S. Department of Defense standards (DOD 5220-22M).
Users can also initiate sanitization while deleting files using the Secure Empty Trash command in the Finder. This command overwrites files as they are deleted using a single-pass erase.
While the Erase Free Space option in Disk Utility offers 7X pass, the secure empty trash feature is only a 1X pass. Tech Republic seems to have conflated the two.