Write a comment or description for an image in Aperture
@terrydev's answer is good if you want to set a lot of photos to the same caption/description.

If you want a faster way of entering individual captions, you can use the "Info" inspector panel (default shortcut is Ctrl+D or via menus: View / Inspector / Info). The drop-down menu at the top lets you choose which set of metadata to display, I think the default "General" one includes caption, version name and headline...

You can type data in any of those, tab between them as usual, and use Cmd+← or Cmd+→ to switch to the previous/next photo in sequence.

This can make it much faster than using the menu + pop-up window for each individual photo. The data will be retained in exported images (if you don't strip metadata on export), and will show up in Facebook/Flickr/etc based on these fields.