Cleaner asks to delete some files. How do I know what these files are? I have downloaded a cleaner app and it works great but it has asked to clean and delete unwanted files and also disable apps. I don't know exactly what all these are:


Cache 
Junk
Residual Files
AD Junk
System Temp Files
Obsolete apks
Memory Junk
Big files


If I click on one of these its going to delete the whole category. What are all these?
Well, each of these things mean something. I hope. Its an app, right? ;)

Well,

Cache is, well, your device's cache. Too frequent cleaning this is a tad counter-active, as most apps will immediately fill this back up. Its like trying to dry a towel. Outside. In a heavy rain.

Junk, Memory junk, AD junk - Stuff you dont need, and is an actual waste of space. Its like rubbish piling up in a bin, and apps will fill this back up, but not very quickly. 
To elaborate:


Junk Is general backfill of files that are not needed often, but may not necessarily be cache. For instance, You may need a backup script once every years its used that rarely, it may be considered junk. Keep in note many apps have different meanings for junk.
Memory Junk is basically the Cache-again. I don't know why, really, it may be a double wipe, which is still a tad pointless
AD Junk is a rarer one. Normally, ad's (or adverts, for those who are not fans of acronyms) are downloaded as needed, then removed from the system after. Some apps, however coughapitoidecough like to clog up the storage by saving every AD (and thumbnail) it has available.


Residual Files are files that were useful, but not anymore. For example, residual files can include your minecraft worlds file AFTER you uninstalled MCPE. Wipe them unless you are planning to re-install the app they belong to.

System Temp Files is stuff like logs and such which can be useful to a modder or debugger, but otherwise can be a tad space-hogging. Never by much though. As the term "temp" suggests, these are files that were of temporary interest to some system app but obviously forgotten to delete afterwards.

Obsolete apks. Oh boy. When Your cleaner app (It sounds like Avast Cleanup, but this still applies regardless) says this, it means apks that you have already used to install an app, so ASSUMES they are no longer needed. If you backup your apps, however, It will probably think they are obsolete and delete them.

Big files are ALL files on the internal storage that pass a certain size, which is either 10Mb or 30Mb (so not that big). Dont worry, this is disabled by default, so you will have to tell it what to wipe exactly here. (Or just tell it to delete all big files, and do a pseudo-factory reset.)

Hope it helps!



Credit to Izzy, who reminded me that obsolete apps can mean backups to cleaners(and polishing the answer) (Izzy did NOT add that bit) and to the Death Mask Salesman, for pointing out how pointless cache cleaning can be.



The answer is tagged as community wiki, so feel free to tweak and modify it!