A user is a username and a password with specific security settings that allows access to the software at the same time as other individuals.  In other words, you can use the software from as many computers and devices as you want, but if you want individuals to be able to work in the software simultaneously, than you need to purchase additional users.  DM can have up to 5 users, DM Pro can have up to 64 users, and DM Pro with SQL can have an unlimited number of users.