There's 2 kinds that I personally experience: 1- Hard stops, vocal cords refuse to make the sound. You know what you want to say, and you can stop the sentence and say something else, but in that moment, your vocal cords refuse to make THAT sound. 2- Repetitions. These are less common for me now compared to when I was younger, and the feeling is similar to hard stops. Your voice gets stuck on a sound, resulting in the stammer that most people associate with stuttering. It's the vocal equivalent of not being able to get a cart over a bump. That's how it goes for me personally, though there's different kinds of stutters that result from different causes.