When I was starting out my career I got asked this in a job interview. It got really tense really quickly because I realised I had no idea. I waffled my way through some crap and people nodded politely. I didn't get the job.
That was a lot of years ago now, but I still don't see a lot of people demarcating between the two. In fact they are used as interchangeably as PR and corporate communications which is a shame because if you get both nailed then you're in a really good place from a communications stand-point.
In its simplest terms, a communications strategy is the big over-arching ambition, the lofty goal - whereas the communications plan sets out the tools you'll use to get there.
So for Draper Cormack Group we have our communications strategy sitting right up the front of our website - "Not just a PR firm, but also a PR firm" - it's our strategy to convey that we are a company who does all the things you'd expect from a PR firm, but we also do more than just public relations.
The communications plan we have for ourselves contains things like "blog your expertise" (must do more often), get well connected so we can make introductions between clients that are mutually beneficial, make sure we trumpet our successes but also own where we go wrong - things like that are what we call the "tactics", to use communications parlance.
If we do all of our tactics and do them well then we will have gone some way to achieving our communications strategy of showing that we are a PR firm, but not just a PR firm.
Communication strategies should be at an organisational level, you don't need to do one for each subsection of a business, or each job/campaign (though they can be helpful). Where they most come into their own is at the highest level so that everyone in an organisation knows that the reason they're communicating in a certain way is to achieve a certain goal.
You can also fold your communications plans and strategies into one document. In a future post, I'll set out what that might look like so you can go away and try and make your strategy yourself. And then if you need a hand, why I know just the communications company that can help you.