Between the optical tube and the mount sits a small ecosystem of adapters, and knowing what each one does removes most of the confusion involved in buying either half separately from the other, which is how experienced observers usually buy.
The dovetail, which is how tubes attach
A dovetail is a metal bar bolted along the tube that slides into a matching clamp on the mount. Two widths are standard, a narrow one for small instruments and a wide one for heavy ones, and almost every mount and tube on the market uses one or the other. That is why tubes and mounts can be bought separately and swapped freely.
The equatorial platform
A driven tilted table that a whole dobsonian sits on. The table rotates slowly about an axis pointed at the celestial pole, so the telescope on top tracks for an hour or so before the platform reaches the end of its travel and has to be reset. It is the standard way to give a large dobsonian tracking without rebuilding it.
The wedge
The fork mount equivalent: a tilted plate between the tripod and the mount that swings the whole assembly over to point its axis at the pole. It turns an alt-azimuth fork into an equatorial one, which is how catadioptric telescopes were adapted for photography before computerised derotators existed at amateur prices.
Counterweights and balance
An equatorial mount balances the tube against a weight on a shaft, and getting that balance right in both axes is the difference between smooth tracking and a motor straining. It takes two minutes, it needs redoing whenever a camera or a heavy eyepiece is added, and it is the most commonly skipped step in setting up.
Questions people ask about equatorial platform
What is an equatorial platform?
A driven tilted table a dobsonian sits on, giving it tracking for an hour or so at a time without changing the telescope itself.
What is a dovetail on a telescope?
A standard metal bar on the tube that slides into a clamp on the mount, in one of two common widths. It is why tubes and mounts are interchangeable.
Why does my mount need counterweights?
To balance the tube about the mount's axes, so the motors move a balanced load rather than lifting the whole instrument against gravity.