Essentially, the difference comes down to apples and oranges -- meaning that while both are in the same general (fruit or Wi-Fi) family, they each have their own unique characteristics and overall benefits.
The Industrial, Scientific and Medical (ISM) radio bands were originally reserved internationally for the non-commercial and unlicensed use of RF electromagnetic fields for industrial, scientific and medical purposes. ISM bands are generally confined to the 900 MHz and 2.45 GHz range, and are used by current 802.11b, 802.11g and Bluetooth devices. These bands are also used by a variety of non-computing devices, such as cordless phones, low-power light bulbs, garage door openers and microwave ovens. All in all, they are busy little bands.
The Unlicensed National Information Infrastructure (UNII) band covers the higher 5.15-5.35 GHz and 5.725-5.825 GHz range and is designed to allow for higher data rates (up to 54Mbits/second). This communications technology is commonly known as 802.11a, which is much faster and more flexible than 802.11b, but is not compatible its less energetic brother. A lot of companies use 802.11a systems for higher-bandwidth applications and for increased security, since this system operates separate from the more available 802.11b and 802.11g systems.
However, the race for acceptance and utility does not always go to the swiftest (to fracture an old adage!). While 802.11a (UNII) is fast and offers a grater bandwidth than 802.11b (ISM) devices, there are some studies which have shown that 802.11a systems suffer more when faced with line of sight (LOS) obstructions. These can be office walls, cubicles, desks and even people (since we are all made up of about 70% water, which is a very poor conductor and an excellent signal blocker). In fact, one study, conducted about a year ago by Intel Corp. involving some 2100 measurements taken over a period of three months within a typical office environment, 802.11b systems faired much better in terms of signal loss and degradation than 802.11a systems. Something to think about when you are mapping out your wireless landscape.