I *SO* accept those terms

Just what are we agreeing to when we sign up for Yahoo Mail or some other Internet account and we check the box that says we agree with some "terms"? Must we quarter some soldiers? Surrender our blenders? What diabolical fates await when we act all agreeable and stuff? Are these free Internet accounts worth giving up something valuable that could be embedded in those wordy-ass "terms of service" or "end user agreement" documents? But who's going to enforce it? The Internet police?