what is .htaccess?how is it useful?

It is a file that lives in the web root of your site.

It can have instructions in it:

- to redirect all non-www requests to www (to avoid duplicate content issues)

- to stop people hotlinking to your images

- to keep certain malicious bots completely out of your site

- to password protect a folder

- to make dynamic URLs look like folder-structure URLs (more friendly to search engines)

- and many other things.