These can be visualized as slightly shorter pieces of type with no image on them. They are used to create spaces between words so that everythingdoesn'truntogether. Each system of type has it's own characteristic design which manifests itself as consistency in line weight, angles curves, proportion, etc. Like ships, many type styles are named. Names such as 'Helvetica', Park Avenue', and 'Hutch' are the names of familiar type styles.
Points are a linear measurement such as inches, feet, and centimeters.
There are approximately 72 points to an inch. This isn't a precise conversion. This system was set up so that printers could calculate how many lines of type they could get on a page. The actual image on a piece of type is going to be something slightly smaller than 18 points tall.