All of these terms refer to values on the pH scale, a standard, scientific scale from 0 to 14 used to measure the level of acid in a substance. Values between 0 and 7 are considered acidic and values higher than 7 are basic (alkaline).
"Acid-free" means that an item has a pH of 7 or slightly higher. "pH Neutral" means that an item has a pH of 7. It is not considered acidic or alkaline. Over time, acids in a material will cause yellowing, staining, and deterioration (think of an old newspaper). Some materials (such as cotton) are naturally acid-free. Wood pulp (used to make most papers) is naturally acidic. Buffering agents are often added to adjust the ph of the paper, making it "pH neutral." This however does not make it acid-free and these papers could still deteriorate or yellow over time if the acid remaining in the sheet or formed during aging exceeds the capacity of the buffering agents.
"Archival" means that a material is suitable for long term display and storage. It should not contain acid or degrade over time. However, this term does not refer to an industry standard and has been loosely used by many manufacturers.
Art Alternatives sketch books are acid-free. They are made from wood pulp that has been buffered to remove the naturally occurring acids.