Accoring to the Safari Help:
Clear the entire history list
- Open Safari if it is not already open
- Choose History > Clear History, and then click Clear.
To clear Top Sites as well as your history list, select “Also reset Top Sites.” The Top Sites page is based on your browsing history.
Clearing your history also empties caches, removes website icons, and clears the list of recent searches. Website icons are small graphics that help identify sites on the Internet. You see them in the address and search field, the bookmarks list, and other places. You can see your recent searches by clicking the magnifying glass at the left end of the address and search field when it’s empty.