Stammering is typically recognised by a tense struggle to get words out. This makes it different from the normal non-fluency we all experience which includes hesitations and repetitions. Commonly it involves repeating or prolonging sounds or words, or getting stuck without any sound (silent blocking).
Sometimes people put in extra sounds or words. Often people Lose Eye Contact.
Some people who stammer talk their way round difficult words so that you may not realise they stammer at all. This avoidance of words, and avoidance of speaking in some or many situations, is an important aspect of stammering. Stammering varies tremendously from person to person and is highly variable for the person who stammers who may be fluent one minute and struggling to Speak The Next.