What is the difference between http and https when accessing Google Reader?

Sorry, I’m not an IT expert, but I am concerned with data interception, since I use a neighbor’s wireless network. If you access Google Reader through https://www.google.com/reader, as opposed to http://www.google.com/reader, what information is encrypted? Is it just my login information, or is all data sent and received between Google and I encrypted?

Any insight into this would be extremely appreciated. Thank you!