A girl who performs live, interactive sex shows via a webcam for viewers from around the world usually on a pay-per-minute basis via a large site with 100s of other performers often referred to as "models". Girls not broadcasting their cams via established sites are called "independent" cam girls and usually charge in blocks of time (i.e. a 15 minute XXX show for $30).
I've been making a living as a cam girl for over 10 years and I love it!
That cam girl was awesome! She rocked my world!