Here is a quick post to answer the question we often get here at Envision Design... "What does it mean when that little thingy pops up on my iPhone?".
There are obvious status bar icons that everyone knows like signal strength and airplane mode, but here is a full list from the iOS 5 user guide (direct link for pdf from Apple) for those times when something shows up and you are not sure what it means.
And yes, I know you are shocked, but there is a user guide / manual for the iPhone. Just click the link above to download the pdf from Apple.