EBNF consists of terminal symbols (literals) and non-terminal production rules (variables).

Here is an example:

Here are what its notations mean:

• :=
• is defined a
• "..."
• terminal string (aka terminal symbol)
• that is, a string literal (as opposed to a variable)
• obversely, non-terminal strings are variables to be replaced with terminal string
• *
• repetition symbol
• (...)
• grouping
• ;
• terminator symbol

References

https://en.wikipedia.org/wiki/ExtendedBackus%E2%80%93NaurForm

http://www.ietf.org/rfc/rfc1806.txt