Exchange-traded stocks have ticker symbols consisting of either one, two or three letters, e.g., F, GM, AOL. If the stock is a preferred stock, they stick a tiny "p" in there so you can tell.
Nasdaq stocks have four or five letters in the symbol; the occasional fifth letter usually indicates something "special" about the stock. CHKP used to be CHKPF, the "F" standing for "foreign."
Related trivia: almost all trading systems on the Street allow six characters for a ticker symbol, "just in case." Every five years or so, some joker comes along and starts a rumor that Nasdaq is going to start using seven-character symbols. Panic ensues, at lease omong those inclined to panic.