A lot of brewers seem to use the terms interchangeably, but there are some important differences.
Cleaning your equipment
Cleaning is always the first step in the brewing process, and it doesn’t matter if you’re making wine, beer, cider or a spirit.
For brewers cleaning means using a specialist alkaline cleaner to remove surface dirt and debris on your kit. If you don’t remove them, they can be the source of bacteria and other unwanted organisms to grow and they’ll ruin your brew.
It’s usually best to clean your equipment both before and after a brewing session.
Why use an Alkaline based cleaner?
Specialist brewing cleaners are usually alkaline based because that’s most effective against the organic oils and starches found in brewing.
Sanitising your equipment
Sanitising brings bacteria (and other organisms) numbers down to safe levels and, in general, is all that’s needed for brewing.
After cleaning, a sanitising solution is sprayed onto the equipment. The solution needs to stay in contact with the equipment for a short time to do its job and then needs to be washed off (unless you’ve chosen a no-rinse solution).
No-rinse sanitisers break down into harmless chemicals that will not affect the taste of your beer, so you can skip the extra step (and save the water).
It’s really important to sanitise any equipment that will come into contact with your brew after it’s cooled down. If you don’t you could be introducing an infection that could ruin all your hard work.
When to sterilise in brewing
Sterilisation on the other hand is the next step up and aims to remove 99.9% of bacteria/organisms. These solutions have a bleaching effect so they’re also used to remove heavy stains. They must ALWAYS be washed off your equipment before you start your brew.
It’s usually a good idea to give your equipment a full sterilisation every once in a while.
TLDR: Cleaning and sanitising are generally all that’s needed for brewing. You don’t need to sterilise unless you want to remove heavy soiling/stains or haven’t used your equipment in a while.