The main difference between Turions and Athlons would probably be that Turions were created to be mobile, and was made for laptops. For example they are low on battery life and so on.
So you can compare Turions to the Intel Centrino technology (well it's not a perfect analogy, but close enough to get the idea I hope). So if you are going to roam around with your laptop then you might want to consider Turion since it has better connectivity and so on. You can read more about it here
On the other hand Athlons are created for processors for laptops that sort of act like desktop replacement, for example like William said gaming and so on. So if you are going to watch movies and aren't going to move around too much then you might want to consider the Athlon in your laptop. You can read more about this here
(notice it has a Mobile version and a normal version).
Oh yeah and by the way, if you only plan on using applications like Microsoft Office, then a Sempron (or Intel Celeron M and Pentium 4/M for some more speed and multitasking) should be fine for you. Semprons/Celerons are in essentially entry level laptops so if you aren't going to do a lot with your laptop then it should be alright for you I guess.
And remember the processor and it's speed isn't the only thing that you've got to consider. Try to get a laptop with about a minimum of 512 MB memory, otherwise no matter what processor you pick it might not be of much use. You can take a look at this thread
as well... there seem to be some helpful tips over there.
Hope this helps
and if you have any more questions fell free to ask... we're all here to help out.