R is number 18 in the latin alphabet. Most experts believe R is derived from an Egyptian hieroglyph that represented a head. It is also the basis for the greek P. In the modern Latin alphabet, the R is unique because it is the only letter with a closed uppercase section, but in lowercase it is open.
The letter R is one of the most commonly used letters in the English alphabet and is commonly added with an E at the end of verbs. This is a great way to add something onto the end of another word to make it easier to get rid of the letter in the game.
The letter R is also commonly used in science and is the symbol of the gas constant. R is also used in mathematics and is the symbol for real numbers.
With 6 R-tiles in the Scrabble letter set, each yielding 1 point, the R is fairly common and worth little points in itself. However it is a common connecting letter for many words, so it is often needed to build high score words. You will want to avoid using it on bonus tiles if you can avoid it though.