Wikipedia : Object Relational Mapping ? :)
To be more precise, an ORM allows you to use Objects directly bound to your Database.
For example, I have a Car, which has 4 object Door, each Door has a Color, each Color a Name.
With an ORM, I get my Car object from the Database. Then I go for a Car.GetDoors(), and I get a collection of my Doors, and for each one of them, I can use a Door.GetColor().GetColorName().
Which means that I didn't bother to write SQL queries to get the Doors from the Door Table, with a CarID, nor get a ColorName from the Color table with a colorID. An ORM thus allows me to write less code :)