PB = Picture book
A story told equally through pictures and words. Average word counts range from 200-800 words, with 0-1000 words generally being the outside lengths most commonly published.
OPB = Older Picture Book
A picture book for older ages with a higher WC (word count) 1500 - 2500.
Books with low WC (less than 300) that encompass shapes, alphabet, animals, colors, numbers and other developmental units for young children
Quick read books with pictures on every page. The sentence structure is simple and exposes children to grammar, characterization & simple plot concepts for ages
CB = Chapter Books
A 'first novel' for children aged 6-10, with pictures and short chapters, usually with a single plot line. Word counts vary from less than 2000 to around 13,000.
MG = Middle Grade
A lengthier novel with few (if any) illustrations. Word counts vary but average 20,000 - 50,000 words and often contain mystery and adventure written for 8-12 year olds. They also contain sub plots in addition to the main one and also include secondary characters.
Ages 12-14, these books often deal with themes that are a bit stronger than MG topics, but not quite YA territory. Tween is sometimes called young YA. The word counts can be either in line with upper end MG or YA, but it is the content and topic dealt with that sets it apart from these two genres—first relationships, friendship issues, family deaths & some coming-of-age.
YA = Young Adult, also called Teen lit.
These books range from 13-18 years of age that can cover stronger topics including drugs sex and alcohol or abuse and violence. The format is much more adult in language and length with complex plots, themes and characters that will appeal to this age group. The word count can range from 60,000 to 80,000 on average.