embed a youtube video in your wordpress site

In YouTube:

1. Click on the <Embed> button underneath the video
2. Uncheck all the tick boxes
3. Select 640 x 505 (or enter custom sizes in the Custom fields)
4. Click on the code and copy (⌘-C)

In WordPress:

1. Edit/create new Post/Page
2. Paste code (⌘-P)

There's more information in YouTube help here!