Unique visitors - Unique IP visits to your website. So the amount of different people visiting your site each day.

Number of visits - Number of times a person has visited your site, so I might visit 50 times in one day.

Pages - Number of different pages visited in a day, same as impressions I believe.

Hits - Number of files loaded, when you visit a webpage and it loads....if it has 50 different image files then that means 50 hits.

Hope it helps.