$ character introduces parameter expansion, command substitution, or arithmetic expansion. The parameter name or symbol to be expanded may be enclosed in braces, which are optional but serve to protect the variable to be expanded from characters immediately following it which could be interpreted as part of the name.
Read more in the Bash User Manual.
Bash is often commonly used in the management and maintenance of servers and clusters. Information pertaining to typical commands used by network operations, when to use which command for which purpose, and examples/samples of unique and/or interesting applications of it should be included