Style guides and related documents sometimes specify a system of capital letter use.

Word processing packages often give four styles to choose from:

• all lower case
• ALL UPPER CASE
• Title Case
• Sentence case

The first two are fairly self-explanatory but here is a definition of the other two common case styles.

Title case – traditionally used for the titles of everything (books, plays, movies, etc), title case has a capital letter for the start of every significant word – where words like and, of, the and a are not counted as significant. {If every word begins with a capital letter, we call it start case.}
The Little House on the Prairie
One Flew Over the Cuckoo Nest
Breakfast at Tiffany’s

Sentence case – just like you use in most sentences, only the first word and any proper nouns start with a capital letter.
The little house on the prairie
One flew over the cuckoo’s nest
Breakfast at Tiffany’s

Sentence case is the default now for most writing, including headings.