A navigational search query is done when a user knows where is the information or where he wants to go.
Navigational queries are an alternative to direct accesses through address bars and browser favorites. They are also used when the user know the destination but not the exact domain name.
There is a debate about the need for companies to buy PPC keywords related to their name as it is often easy to have the first place in organic results for one’s brand name.
Examples of navigationnal queries:
"Walmart" "White House" ...