What is the best way to check my iMac for virus, malware...

If your computer has the Apple logo, it does not have a virus.

It is possible to get malware from unscrupulous sites that trick you into giving permission to download. The ONLY third-party protection most of us long-time contributors recommend is MalwareBytes, written by an active contributor here. Get it ONLY from the developer's site:

Free Antivirus Replacement & Anti-Malware Tool | Malwarebytes

Do NOT install anything else that claims to clean, defend, protect, optimize, or find your Mac a mate. The Mac OS is engineered to take care of itself and all those fearware products add nothing but almost always extrace a seriuous performance penalty.

I have an iMac (OS High Sierra) and have just completed the free trial with MalwareBytes. It definitely slows things down, but that's OK if I can be confident that my computer is protected. Before I upgrade to a paid version I'd be grateful for answers to the below.

Is there a difference between viruses and malware? This may be a naive question, but if you don't know, you don't know! if there is a difference, how can I best protect.

And, whilst asking all these questions, do I need to protect separately my iPhone and iPad?

Thanks

Listen to Allan, he's absolutely correct!