Edit: Add-ons don’t appear to be working yet, at least in the UK. The ‘Add-ons’ drop-down in the menu bar doesn’t appear, and clicking to access anything from the store just opens up a blank doc/sheet. Damn you Google!
Add-ons basically give Drive more Office-like functionality, such as mail merges and bibliographies, useful features like approval workflows, as well as cool partnerships with tools like MailChimp and Supermetrics reporting. Nice. We’re always keen to use the online features of Drive to automate our processes, and this hopefully is the first step in making this type of usage more accessible and easy to set up.
See the announcement here: Google announce Drive add-ons