T-mount is an old mount style where the mount was interchangeable allowing you to use different mounts on the lens allowing it to fit different cameras. True T-mounts have been extinct for 40 some odd years now having been replaced in the early '60s with the T2-mounts. T2-mounts are interchangeable with T-mounts and add the ability to loosen 3 screws to align the top of the lens with the top of the body. All T-mount lenses require either a T-mount or T2-mount camera mount. These are usually incorrectly called an "adapter" though they don't adapt anything; they are a removable mount. Today, its only use is for telescope mounts and a few odd camera accessories (slide duplicators, microscope adapters, ...) left over from "olden days".
Here's a short list of the more common antique mounting systems:
T-mount, T2-mount: no difference in the lens, only the rotational difference in the mount. These lenses are either real manual aperture (it doesn't automatically close when the shutter fires and reopen after the exposure) or pre-set (also not auto-diaphram but you have an open/close ring in addition to the f/stop ring). These mounts screw onto the lens.
YS-mount: a scarce old auto-diaphram update on the T/T2-mount system; same thread.
T-4: a Soligor and Vivitar exclusive. These mounts used bayonet fittings, breechlock style like the old Canon FL/FD mount, and carriy full auto-diaphram and meter coupling. Mounts were available for most '60s and early '70s vintage cameras.
TX: and update to the T-4 mount that added some additional mounts for newer camra models that were incompatible with the older T-4 system. Many of the TX mounts were identical to the T-4 version. All T-4 mounts fit any TX lens but one some of the TX mounts will fit T-4 lenses. More cameras from the '70s, like the Pentax bayonet models, were added
Adaptall: an old Tamron system that used a screw on retaining ring. These have been extinct since the mid to late '60s.
Adaptall 2: the newer Tamron system that was viable through the '70s. Unfortunately humans are generally verbally lazy and few comments on forums and few
eBay-style venders use the proper name for these, generally incorrectly labeling them just "Adaptall" omitting the critical "2". The two systems are
not interchangeable. Mounts were available form most '60s and '70s vintage models but not for mounts introduced more recently (e.g. Minolta Maxxum, now Sony, and Canon EOS).