A blog, shortened from "web log", is a website written by an individual or a small group about a particular subject. Blogs can be professional or personal, and often contain images and links in addition to writing.
A book is made of many sheets of paper or parchment, bound together down one side. Each sheet is called a leaf, and each side of a leaf is called a page. Books can have lots of pictures or just words. They might have a story in them that is all true (non-fiction) or a story that someone has made up (fiction). Reading books is a hobby a lot of people enjoy.
A chapter is a section of a book that usually has a number or a title. Chapters organize books into parts.
A diary is a blank book in which you write about what is happening to you. A similar word to diary is journal.
A letter can mean one of two things. A letter is either a letter in the alphabet, like the list of letters at the top of this page. Or a letter is something you write to someone, put in an envelope and then send in the mail.
A novel is a type of book that has a fairly long story in it, that can take hours or days to read. Novels usually contain made-up stories or true stories that have been written to be more interesting. People generally read novels for enjoyment.