A) "Too long; didn't read.", meaning a post, article, or anything with words was too long, and whoever used the phrase didn't read it for that reason.
B) Also used by someone who wrote a large posts/article/whatever to show a brief summary of their post as it might be too long.
Guy One: Did you read that book for English class?
Guy two: No, tl;dr.
Guy one: Cake is a form of food that is usually sweet and often baked. Cakes normally combine some kind of flour, a sweetening agent (commonly sugar), a binding agent (generally egg, though gluten or starch are often used by vegetarians and vegans), fats (usually butter, shortening, or margarine, although a fruit purée such as applesauce is sometimes substituted to avoid using fat), a liquid (milk, water or fruit juice), flavors and some form of leavening agent (such as yeast or baking powder), though many cakes lack these ingredients and instead rely on air bubbles in the dough to expand and cause the cake to rise. Cake is often frosted with buttercream or marzipan, and finished with piped borders and crystallized fruit.1
Cake is often the dessert of choice for meals at ceremonial occasions, particularly weddings, anniversaries and birthdays. There are literally millions of cake recipes (some are bread-like and some rich and elaborate) and many are centuries old. Cake making is no longer a complicated procedure; while at one time considerable labor went into cake making (particularly the whisking of egg foams), baking equipment and directions have been simplified that even the most amateur cook may bake a cake.
tl;dr: Cake is a baked, yummy sweet.