Picture books or PB - are for the youngest of children, usually under 8 years. Manuscripts are up to 1,500 words, but 1,000 is average. Page wise they run 6-9 typed pages. When put into book format they run either a 16 or 32 page format. Plots are simple and the illustrations, which are on every page, play a large role in telling the story.
Chapter Books- these books are for the next level of readers, ages 7-10. They fall into two categories- early chapter books and regular chapter books. Early chapter books have manuscripts that are up to 30 pages long and are broken into 2-3 page chapters. For example, Nate The Great books by Sharmat. Regular chapter books run 45-60 pages and are broken up into 3 or 4 page chapters. These books usually are full of action and have more complex sentence structures, but still have short paragraphs. Check out the Ramona books by Cleary.
Middle Grade or MG- This is my sweet spot. Middle grade books are for children ages 8-12. The manuscripts run between 100-150 typed pages and can run between 20,000- 50,000 words. At this age children are ready for more complex stories, subplots and secondary characters. Middle grade books often come in series because children at this age get hooked on characters. A great example of MG is Holes by Sachar.
Young Adult or YA- For ages 12 +, these works run between 130-200 typed pages and are usually between 50,000-85,000 words. Plots are complex, issues sometimes extreme, and there are several major characters and many subplots. Themes often are relevant to what is facing teenagers of our day. Check out Matched by Ally Condie.
Cross-over - These books are becoming more and more popular with the release of works such as Harry Potter and Twilight. Manuscripts are like YA, but the plots are often interesting to adults and can be sold in both the YA and adult market.
Just a side note- there are no specific requirements for any type of book. The above information is more of a guideline. You'll find books that break the norm in every genre.