The most common Wi-Fi wireless networks (802.11b or 802.11g) operate in the 2.45 GHz range -- oddly enough the same range populated by a variety of non-computing devices, such as cordless phones, low-power light bulbs, garage door openers and microwave ovens. This sometimes results in some degree of signal collision and degradation -- although nothing you'd usually write home about. The fast and wider bandwidth 802.11a system operate in the 5.725-5.825 GHz range, but are not compatible with 802.11b and 802.11g systems. This is why it is not uncommon to see most companies juggling both 802.11a systems (for more sensitive data) and 802.11b/g systems (for more everyday and less mission critical information).
Go on to part 4: The range of wireless networks