The tv-series is more focused on children than the movie. It took the right to include some freedoms. Let's say, that the first two seasons follow the original story relative close. The third season is something like a sequel and has nothing to do with the books.
The second season also ventured to do some bigger story changes, that I don't want to spoil here.
Because I'm a huge fan of the series- I created a little fan-site, called
HighLonelyHills.com - just have a look and I think, you find everything you want to know about the tv-series

But be aware: A look in the episode guide may reveal some interesting twists. So be careful while reading it. I think, the page
About Watership Down could give you a good overview about the series.

Edit:
IMPORTANT: don't be putted off by the first episodes. Trust me, it's really worth watching the whole series, because season 3 is the best of them all! But for understanding, you need the first two seasons also, of course.
Edit: Oh, and what I forget the say: The second season has some fillers. When you look at my site, I've rated some episodes with 1/4 points. Simply, because I created that overview of grades. Don't worry. Even the worst episodes (keyword sheeps & visitors) are really viewable and worth watching! 1/4 on my site means 8,5/10 on a normal ranking system (at least on the episode guide)
