I'm not an 'expert' on anything!
If you download the free version of Malwarebytes Anti-Malware you automatically get a time-limited trial of the full 'Premium' version. If you do nothing that reverts to just the free version after the end of the trial.
The difference is that the Premium version works 'in the background', meaning that it's always running and on the look-out for anything nasty, wheras the free version doesn't do anything at all unless you actually run it. (i.e. it's 'run on demand').
Further, the latest Premium version is designed as a
replacement for your current anti-virus program, rather than as a supplement to it (as the free version is).
I've actually taken a bit of a dislike to Malwarebytes Anti-Malware recently though, as it forces you into trying the Premium version and (here's the important bit) I've found that the Premium version can severely slow some of my machines down as it runs in the background.
My own preference is to stick with my current anti-virus program (which is Avast) and to supplement it with AdwCleaner, which is an alternative to Malwarebytes Anti-Malware for zapping adware, etc.
AdwCleaner used to be entirely separate to Malwarebytes but is now owned by them, and should therefore be just as good. Just like the free version of Malwarebytes Anti-Malware, it's 'run on demand' only, so you need to remember to run it every so often. (I suggest once per fortnight or, of course, as soon as you seen any signs of possible malware appearing). However it's not linked to any time-limited trial of a 'premium' product, and it doesn't 'run in the background', so you don't need to worry about it possibly slowing your computer.
https://www.malwarebytes.com/adwcleaner/