Stores and clients dictate what data is coming into Learning Record Store and where it is stored.
A client is the point at which data arrives in Learning Record Store. Each client creates authorisation details which you use to set up your learning platform to send statements to Learning Record Store. You can create as many clients as you like and can have as many learning platforms sending statements to each client as you like. It's best practice to use different clients for each learning platform if you are receiving data from multiple platforms. It makes it easier to locate problems and to filter your data. Clients also decide which store your data should be stored in.
A store is an empty container for data to be stored in. You can create as many of these as you like and you can have as many clients feeding into a single store as you like. Again, it is best practice to use different stores for different learning platforms because you can filter your data by store in the query builder.