they're physical bitcoins which can be redeemed for a certain amount of bitcoins. some people likes to collect both funded and redeemed casascius coins.
btw OP try using google before creating a topic next time
this is a forum for me to ask questions like this google aint no bitcoin forum.I wanted a thread to see peoples ideas about what a casascius really is u dont know who you are helping with your post thanks for the help tho