Using Roman type will improve the readability of your copy because the serifs make it easier for the eye to distinguish characters. Sans Serif has no serifs. It's great for type in columns, headlines, and on contracts where the type gets very small. Text recreates the feeling of medieval scribes and is used for formal situations such as invitations. They are almost unreadable when all uppercase letters are used.
Script recreates the image of handwriting.
But don't use it all in uppercase. Occasional is pretty much anything that doesn't fit into the other categories. This could be special type created for a logo or something of a novelty nature. Leading refers to the amount of space between lines of type. The larger the type, the more leading necessary.