The video transmitter (or “vTX” as it’s usually written) takes the video signal from your camera and transmits it wirelessly to your goggles. The single most important function of the video transmitter, to me, is how easy it is to change channels and transmit power. FPV video transmitters operate like old analog television signals. (That’s an analogy, but it’s also literally true! Your quadcopter is a tiny television station!) If two transmitters are on a channel too close together, they will interfere, and both pilots won’t be able to see to fly. When you fly with other pilots, you’ll have to organize who is on what channel. If you can’t quickly and easily change channels, that becomes a hassle. You’ll be “that guy” who everybody else has to make room for.
Second, they come with different connectors. Video transmitters typically have U.FL or MMCX connectors, due to their small size. These may plug in to a “pigtail” wire that ends in an SMA or RP-SMA connector. For racing, it’s most common to buy an antenna with an MMCX or U.FL connector, because it saves size and weight compared to a pigtail. These antennas are typically held to the frame with 3D printed accessories. Keeping the antenna close to the frame vs. out on a long stalk shortens the range of the video signal (not really an issue for racing) but increases durability. The antennas shown on this page are all MMCX and UFL, but if you need an SMA version, they are also made. If you are buying an MMCX antenna, it’ll almost always work best to get a right-angle MMCX, as having the connector come straight out of the vTX will usually not fit as well inside the build (but every build is different).