I am a new member, want to know what are credits and what do we do with credits????????????????????????

They are nothing. You get nothing. Just for fun, comparing how much you have done to others.
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Doesn't expecting the unexpected make the unexpected the expected?
If it makes sense, DON'T do it.

Maybe some graphics artist may be able to make some boinc credit banknotes, so you can print them out! Instead of presidents have scientist printed on them, etc :P