Regular Marijuana seeds can grow into either male or female plants. The ratio is about 50/50. Please keep in mind that's an average. Sometimes you'll get 2 females only, or if you're in luck, 9 females and a single male. Usually, you'll end up with roughly the same amount of female plants whether you buy feminized or not, because feminized seeds come 5 to a pack.
Regular cannabis seeds are generally considered to be stabler and less prone to hermaphroditism. A feminised plant that is stressed (for example by light leaks, too much heat or cold, or by being replanted too often) may develop male as well as female flowers. Generally speaking, the choice between regular and feminized seeds is a personal one. Either option could suit you, depending on your experience, grow room facilities and personal preferences.