gnus newbie question: why do read articles disappear from summary?

[Brendan Miller]

After reading an article in summary it gets marked with 'R'. If I leave
the summary buffer for my group with q and log back in, that article
disappears. Why is this? Also, is there a way to prevent this behavior?

I really only want articles to disappear if I manually ask for it.

Also, I've noticed there is a way to mark an article read using d, but
it makes it with a lower case 'r' flag. What's the difference between
this lower case 'r' flag and the upper case 'R' flag?

Thanks,
Brendan Miller