What's a "labour of love"?

From Princeton's WorldNet:

productive work performed voluntarily without material reward or compensation

Google definitions

Example usage:

Grandma's sweater for you was a labour of love, and yet you refused to wear it! She spent hours knitting it, and you threw it away like an old gym sock! You should be ashamed of yourself!

As you can see, "labour of love" is a selfless act.

It's a labor performed for the love of the work or for the target of the finished product. From TheFreeDictionary:

a labor undertaken on account of regard for some person, or through pleasure in the work itself, without expectation of reward.

In some ways it's similar to the concept of pro bono work (work done for free), but it normally is used to describe work someone really enjoys doing and is not compensated for.