scrapbook is where you can exchange messages with another RxPG member. It can be a public message i.e visible to everyone or if you tick a box, you can share a message in private.
Messages can also be sent via PM (private message) which also features on a member's homepage or under the post made by RxPG members. PM as the name says is 100% private.
The difference between the two is that, if you scrap a person, he/she gets notified via his registered mail. However, PM's can only be viewed when a member login his RxPG account.. so for those who are not regular visitors, its better to scrap with the hope that they check their mail accounts regularly.