"travel" usually refers to a nurse who travels more than 50 miles from home to work, and works for a travel company that, in addition to hourly pa, provides housing, insurance, and other benefits. As a travel nurse, you are usually guaranteed 36-48 hours a week for an 8-13 week period (usually 13 weeks).
"Per diem" means you work for a staffing agency, and are usually paid more because they usually don't provide benefits like insurance or housing. You are also not usually guaranteed hours, and may be requested to work at different facilities from one week to the next.
It's hard to quote actual pay rates, because it varies so much based on where in the country you're working, benefits you'll be getting, your specialty, certifications, experience, what shift, and the needs of the facility....