Re: aikido, judo, jiu jitsu, hapkido
There are many differences. There are philosophical differences, training method differences, and in the case of hopkido, culture differences.
I've never taken hopkido, so I can not comment on it. But I would guess its aikidoish with more punching and kicking.
Aikido is usually (with the exception of some styles) strictly about redirecting your attacker to the ground. Very little focus is given striking (although its importance is usually implied ) and you will spend time focusing on how to redirect and blend with a persons movements as opposed to just punching them in the face and eye gouging them. There can be a heavy focus on building ki energy, sometimes weapons training, and in some smaller circles there is some competitions. Usually however it is a very circular flowing almost dancing type movements followed by abrupt stops causing the person to fall down. A common movement would be stepping to the outside and guiding your partner in a circle as he tries to grab your wrist, leading him in a spiral pattern that leads to a throw.
Jiujitsu is a varied beast that is usually going to have a focus on striking, throwing, and joint locks. It is going to be more 'always kill your attacker as fast as you can'. Like aikido, some styles will talk about ki, some won't, some will be physical, some won't, some will have weapons, some wont. It is very varied. Saying jiujitus is a lot like saying linux. There are a million different kinds, but they are basically the same. They usually focus on being a complete fighting system. A common movement would be a punch to the nose followed by a kick to the inside leg then a throw over the back finished with a wrist lock.
Judo is usually going to have a sport feel to it with lots of cardio/strength training and sparing. You will spend the majority of your time sparing, and will not learn to use strikes outside of some very silly katas. You will however learn how to throw someone very well and very quickly as this is the main focus of judo. A variant of judo (well more like a distant cousin) is Brazilian jiujitsu, basically again heavy focus on sparing with more relaxed rules. Still not striking training however and more emphasis on the ground then the throw. Ideally I believe these two must be trained together to get a complete grappling system. They are two halves of a whole. Still the lack of striking should be a concern if you are looking at self defense issues. This can be solved with some supplemental training (which takes you down the MMA route). A common judo movement would be fighting for grips on the gi followed by a unbalancing technique (usually a turn, push, or pull) that lifts your partner, at this point you would pivot in and throw, if this was bjj you would then secure a position on the ground and attempt a large joint lock (elbow, knee, neck, ankle, etc) or choke.