I will be honest I did find this series hard going in places. I didn't enjoy Shiver (the first in the trilogy) as much as the other two which is unusual for a series.
The books are about a pack of werewolves who change to wolves in cold weather and do not turn back until they are warm enough which basically means they are wolves throughout winter and humans during the spring and summer. This transition continues until they have been werewolves for around 15 years then they stay wolves forever.
The main characters are Sam (a wolf) and Grace (the girl who loves him). We also meet Cole and Isabel later in the trilogy.
The book is written from the perspectives of all of the main characters. Which is very interesting but also can become confusing at times. I also would like to add that the chapters are all very short which I like as I like to stop reading at the end of a chapter.