There are several WiFi protocols known by their letter designated IEEE 802.11 specifications, such as 802.11a, 802.11b, 802.11g, 802.11n, and 802.11AC. The differences are related to frequency (aka band or spectrum), data rate (aka throughput or transfer speed), bandwidth, modulation and range.
Bandwidth is often confused with data rate. While there is often a correlation between greater bandwidth and greater data rate, in terms of radio the bandwidth refers to the difference between the upper and lower frequencies of a given channel as measured in hertz. For example, with the 802.11b and 802.11g protocols the first channel will have a lower frequency of 2.400 GHz and an upper frequency of 2.422 GHz for a total of 22 MHz bandwidth, however the 802.11g protocol uses a more advanced encoding scheme allowing for significantly faster data rates in the same amount of bandwidth.
Modulation also affects data rate, with the most common modulation types being OFDM or Orthogonal frequency-division multiplexing and QAM or Quadrature Amplitude Modulation. In addition to being a mouthful, these are digital encoding techniques used to cram a lot of data on a small amount of spectrum. Typically newer 802.11 WiFi standards offer either improvements to the encoding scheme, or entirely new encoding schemes.
802.11a and 802.11b were the first mainstream WiFi protocols, introduced in 1999. 802.11a operates in the 5 GHz band with speeds up to 54 Mbps while 802.11b operates in the 2.4 GHz band with speeds only up to 11 Mbps. Today, these networks are more rare to find, though when they are it’s typically indicative of aging infrastructure.
Modern networks are usually 802.11n and 802.11ac, with data rates as high as 1800 Mbps, though typically lower speeds are actually observed. As the WiFi standards evolve and new products make their way into the marketplace, the common devices evolve.
Typically older devices are still able to use more modern access points via backwards compatibility: An 802.11n device can typically connect to an 802.11ac access point, but will only be able to do so at 802.11n speeds. Not all newer standards are backwards compatible with all devices, however.