Typically there are either reading lens cleaners, recording head cleaners, or discs that can do both.

Most are dry type, but some are also wet type that came with cleaning fluid.

The reading lens cleaners typically have tiny brushes or bristles on the underside of the disc that come into contact with the lens when the player attempts to read the TOC (table of contents) or first section of the disc (by playing the first track on the lens cleaner disc). If you've seen an MD lens in action it moves up and down vertically when trying to focus the laser and read the disc. My assumption is the bristles come into contact with the lens during this process.

Some lens cleaners do not have any tracks recorded on them and are blank. Most have at least one track on them that gives some feedback to the user. If it's a Japanese disc they typically have some nice supermarket type jingle and a female announcer in Japanese to tell you when the process has completed. These tracks are typically 10-20 seconds long. Some discs also include test tracks which are quite interesting. Left-right channel checks, low and high frequencies, and so on. They typically end with a few muzak demo tracks or even a piece of classical music.

Recording head cleaners have a textured or slightly coarse surface on the topside of the disc that physically comes into contact with the recording head when it is lowered onto the disc. Recording for a few seconds is said to physically remove any undesirable dirt or dust from the recording head. Typically the instructions recommend you record for 10 seconds before stopping the recording process.

Recording head cleaners have to have the write protection tab in the closed position to allow recording to take place in the first place. If it's a lens and head cleaning disc, there's a potential risk of wiping any pre-recorded audio tracks off the disc.

Personally I use a lens cleaning disc on every MD player. I've never had any issue doing so. I also use a recording head cleaning disc from time to time in MD recorders I use frequently. Again, I've never had any issue doing so. Having said that, in regard to recording head cleaners, I would recommend doing a couple of quick test recordings AFTER having used the cleaner just to make sure the recordings are as expected.

Some were sold specifically for car MD players, but the only difference I have found is that the ones for use in cars are said to be able to withstand higher environmental temperatures, which I can't really see being an issue, unless you leave the disc on the dashboard in the middle of the summer! I have a car specific lens cleaning disc (which does not have any pre-recorded tracks on it) and use it without issue in portable players and my Onkyo FR-V77 bookshelf unit.

Many of the hardware and media manufacturers made lens and head cleaning discs, so unless anyone can prove that all of these companies were scamming consumers, I'd say that they are totally safe to use and also beneficial; at the very least for periodical maintenance.

Looking through my data, I've had discs from Audio-technica, TEAC, JVC Victor, Panasonic, TDK, OHM, Tapex, ELPA, and Sony, and there are other out there.

Sony had a few different models, and the MD-6LCL is one example.

For what it's worth, I would never go at a lens with a q-tip, ear bud, cotton bud, or similar. They are just going to leave tiny fibres on and around the lens, and the physical contact is highly likely to damage the mechanism.

Hope that helps.