Complex sentence: Doorway pages are web pages that are created for spamdexing, this is, for spamming the index of a search engine by inserting results for particular phrases with the purpose of sending visitors to a different page.
Split into simpler sentences: Doorway pages are web pages that are created for spamdexing. This is for spamming the index of a search engine by inserting results for particular phrases with the purpose of sending visitors to a different page.