Posts in reddit are partitioned into "subreddits". Each submission is made to one subreddit, though multiple submissions can be made to spread items to different communities. Each subreddit is essentially like Hacker News, a list of ranked links with comments all flavoured by some common theme. The front page of reddit aggregates posts from your preferred subreddits (or a default list of popular subs for users who aren't logged in).
/r/askscience is a subreddit in which users can ask real scientists questions about science. It's a less-structured StackOverflow, essentially. Submissions are somewhat moderated, comments are more heavily moderated. Top-level comments are not allowed to be jokes, and offending comments are removed. On-topic jokes in other places are allowed, but too much joking around in a thread will often lead to all of the comments in the thread being removed. Deleted comments are replaced by "Comment removed" tombstones, and seeing whole trees of these tombstones is a decent reminder to stay on-topic.
The CSS of the subreddit also encourages the community to maintain their standards. When upvoting, downvoting, commenting etc, users are reminded what those actions represent in the community. There is also an informal hierarchy of posters, with experts' comments being identified with short descriptions of their specialisations. The posts of these users are given more weight by the community, but the posters are held to somewhat higher standards.
/r/askscience is a heavily moderated subreddit where there are many rules for posting/commenting beyond those of other subreddits. Top level replies that don't site verifiable sources or contain layman's speculation are deleted. Comments that are off topic or do not comply with the rules are also promptly moderated.
repsilat and ceph_ are right, but I'll add that it's noteworthy that askscience is good. Askscience is a default subreddit, meaning that everyone who joins reddit sees it and is drawn into the conversation. Large subreddits with lots of newbies tend to be... problematic.
