new to 4chon. can someone show me around?

new to 4chin. i mean to explore about 4chan as it is very popular. need some guidance esp the topleft corner "B" which i understand to be for various board. but why not use word "board" instead of word "B". and why so many boards? is there a summary of all the boards?<br> or if u can show me around, what are some interesting things that u can share with new user like me.<br><br>tks