.com is for international for-profit companies, officially, though many of them belong to just american companies given the place the web really took off first.
.co.uk is for for-profit companies in the UK.
Both are used for other purposes too though, especially .com, given the fact that it's easy to remember ("dotcom").
As fo3nix said .com is used globally but is mainly used by americans. So in answer to your to your question if your planning on going for a global audience then .com. British people will only type .co.uk so obviously if Britain is your target audience then .co.uk is better
An advantage for .com is that the user only has to type the domain name in (eg google) and then hit CTRL + enter and it automatically puts the "http://www..............com" around it. I don't think you can do something like that for .co.uk, but you can do it for .net by pressing shift +enter and .org by pressing CTRL+shift+enter