Reddit is the link aggregation website, and each user can upvote or downvote a link. Reddit is very popular and is used by 174 million users. Subreddit is like a group inside reddit where similar minded people may gather and discuss on that subreddit topic.
Besides the default subreddit, which I think is good, I would recommend the following subreddits.
http://www.reddit.com/r/suggestmeabook/ – Book suggestion, answers are given very fast due to the high number of users, can crowd-source answer.
http://www.reddit.com/r/compsci/ – Computer science
http://www.reddit.com/r/artificial/ – Artificial intelligence
http://www.reddit.com/r/Futurology/ – This is for people interested in the future, especially exciting advances in science and technology.
http://www.reddit.com/r/productivity/ – You can view the top answers for top productivity tips.
http://www.reddit.com/r/socialanxiety/ – special one for me, I have mild/ severe social anxiety, if you have any other mental illnesses, feel free to join the specific subrreddit.