The A, B, and Z forms of DNA are all duplex (double-stranded) structures, all with Watson-Crick base pairing. Triplex forms are three stranded, in which two are generally held together by Watson-Crick base pairs, and the third uses the Hoogsteen edge for interactions.  G-quadruplex DNAs are four-stranded, with each guanine forming Hoogesteen interactions (using both their Watson-Crick edges and their Hoogsteen edges).  A comprehensive overview of DNA structure and descriptions of these forms can be found in the book chapter "DNA Structure: Alphabet soup", which is open access.

http://www.intechopen.com/books/dna-replication-current-advances/dna-structure-alphabet-soup-for-the-cellular-soul