What constitutes an 'electronic signature'?

When I signed up for an MSN passport it says this:

What exactly are they asking me to agree to? I have no idea what ‘electronic signature’ is.

An electronic signature is a method of taking a given message, along with a user’s private cryptographic key, and generating a digest code that verifies that the message that was received is exactly the same as what the person who signed it sent.

Think of it as a “tamper-evident seal” on a message.

It’s hard to prevent a message from being altered after it is sent. But it’s relatively easy to give a recipient a means to tell whether or not the message arrived intact.

But how does MSN know his public key? Did it create one for him when he signed up for this Passport thingy?

I read this same warning about 1.5 years ago when I got a new computer. It came with McAffee Virus Scan, which would give you free updates if you registered with them, but you couldn’t register with them without an MS Passport. I read their little agreement and decided I’d rather have outdated virus definitions.