Wireless computer network equipment typically uses radio signals in either a 2.4 GHz range or a 5 GHz range.

The GHz range of a wireless radio is only partially related to the speed of a wireless network. For example, 802.11a wireless operates at 5GHz and 802.11g at 2.4GHz, but both support the same maximum data rate of 54 Mbps. However, newer 802.11n and 802.11ac routers have the capability of simultaneous dual-band operation on both 5GHz and 2.4GHz ranges, allowing clients to connect on different bands for more flexibility and less interference. 802.11ac specifically expands on that, running entirely in the 5GHz band.

Advantages of 5GHz:
The 5GHz band is less likely to be congested. The 2.4GHz frequency range is much more prone to interference, as it is commonly used by other wireless networks in the area, as well as cordless phones, garage door openers and other home appliances and consumer products. The 5GHz band can also offer much higher throughput (using the right technology) with the same channel width. It has 23 non-overlapping channels vs. only 3 in the 2.4 GHz band. 802.11ac in the 5GHz band implements many newer technologies, such as, MU-MIMO, beamforming, etc.

Disadvantages of 5GHz:
In general, the higher the frequency of a wireless signal, the shorter its range. Thus, 2.4GHz networks cover a substantially larger range than 5GHz wireless networks. In particular, the higher frequency wireless signals of 5GHz networks do not penetrate solid objects nearly as well as 2.4GHz signals, limiting their reach inside buildings with solid walls and floors. Recent 802.11ac devices, however, are able to mitigate some of this disatvantage by using beamforming.

The Bottom Line:
5GHz and 2.4GHz are simply different frequencies, each with its advantages and disadvantages. To get the best of both worlds, some recent routers have the capability for dual-band operation in both ranges simultaneously. 5GHz offers higher throughput at a shorter distance, while 2.4GHz offers increased coverage and higher solid object penetration. Beamforming and other newer technologies allow 802.11ac to achieve cleaner signals with 5GHz in many situations where the 2.4GHz spectrum is congested. The migration to wider adoption of 5GHz will probably continue with the shift to the higher throughput 802.11ac.

When choosing the band for your router, it is also important to consider the capabilities of your wireless clients. Many laptops, tablets and other typical wireless devices still work only in the 2.4GHz band, while media streaming devices and newer phones have increasingly better adoption in the 5GHz band as well.

802.11ac routers/access points support 802.11n in the 2.4GHz band in addition to 5GHz devices, so you get two separate radios that can be used simultaneously.