WiFi antennas are those which operate effectively on the IEEE 802.11a/b/g/n/ac/ax frequency bands, most notably 900 MHz, 2.4 GHz, and 5 GHz. WiGig antennas (those operating on 802.11ad/ay) can be instead found under the mmWave section.
Good quality WiFi antennas are those tuned to provide high efficiency and low VSWR across the specific WiFi operating bands. Modern WiFi antennas must also be dual-band, where they must perform well on both 2.4 and 5 GHz frequencies.
WiFi antennas used in a typical indoor environment are most commonly microstrip dipole designs which receive and transmit in a spherical “bubble” pattern. This allows them to communicate effectively regardless of their location or orientation.
Almost all WiFi equipment these days use multiple antennas, known as MIMO, to provide even more resilient communication, and significantly faster data speeds. This can either be as one single MIMO antenna, or multiple separate antennas.