Others will soon chime in much more detailed, but if you already have 2 track equipment you don't wanna get into Maerklin 3 track stuff. Today it isn't really AC, it's just called that because it avoids confusion (of course there are still lots of Maerklin users with the old AC transformers) but when digital both 2 and 3 track use the same power system.

The difference is that Maerklin have a pickup shoe that gets power from a middle rail so that both tracks for the wheels have a common ground. Whereas 2 track has power in one of the tracks and ground in the other.

Roco, Brawa, Piko, Esu and what else have lots of 2 track locomotives, they'll usually be called DC (after the old system with direct current/ positive/negative) even though they run digital DCC. These brands also offer 3 track locomotives, usually named AC. Also Maerklin have their Trix line which is 2 track.

As a side note ESU locos often come ready made for both 2 and 3 track because the slider can be removed so that the engine switches from 2 track to 3 track. For other brands you'll have to buy a specific AC or DC locomotive.

On locomotives 2 track types will have wheels/axles isolated from each other. 3 track will have unisolated wheels so that the current from the middle shoe can flow through the decoder and motor to both sides. Cars and waggon for 2 track must have isolated wheels in order not to short circuit. Cars and waggons from Maerklin will usually have unisolated wheel sets, but you can run rolling stock from 2 track companies (like Roco, Brawa etc.) on Maerklin tracks.

Hi!
These days manufacturers usually write "DC" for two-rail locos and "AC" for three-rail locos.
All "AC" locos produced these days are digital, but also work on analogue layouts. "AC" on the box of rolling stock means "for the Märklin system", including C track. Older items may be analogue.
Locos for "DC" are not always digital, but usually are "digital ready" and a decoder can simply be plugged in.

Many folks used Märklin tracks for DC operation, but I don't think that locos for that type of operation were ever massproduced.

Avoid Trix Express locos - that's a completely different three-rail system.

With respect to cars and coaches: so-called "DC" wheelsets from Piko and Liliput will derail on Märklin C track, so make sure to get "AC" wheelsets for them or adjust the wheelspacing.
So-called "AC" wheelsets will short circuit the "DC" tracks.

Thank-you STE-BM, you and everyone on this site are very sympathetic and helpful! I am glad to be part of this forum and hopefully one day, I will know enough to be able to give some advice to others myself. I did see some posts about conversion of DC to AC lokos, yes. It seems like the lokos that I like are usually available in a format that I can use though. Thanks again.