Evil. Dont use them. Blackhat. They are basically pages that a short, sharp , and the 3rd one. Jump pages are used just to rank well in search engines. They are HIGHLY keyword optimized making the search engines think that the rest of the site, in a way, is like that. I personally do not like these and don't use them but there are many people that do. There are many more that do it in a 'friendly' way by just not spamming the keywords within the page but I still dont like doing this. A doorway page can also be a sperate site for search engines an the visitors. Using search engine IPs to show search engines extremely content rich pages wheras real visitors are shown something sperate. If google comes across you using these you can say Bye Bye cos ur goin Fly Fly. I am not sure 100% what google's standing with jump pages though.