Can anyone explain to me what long lining is and how it is different than longeing? It seems complicated if not dangerous when I saw one person doing it. Does it have a benefit beyond longeing? Is it easy for someone to pick up? what equipment do you need? have any books or videos you'd recommend that describe it?

I am assuming you are referring to ground driving. There are many benefits to doing this, especially for young horses. This is a great way to get them used to the bit and learning to respond to cues when they can't see you. Most of the time when you are training young horses, you are right by their head teaching them to lead, longe, etc. Driving teaches them that they have to respond to your cues even when they can't see you. It is not really dangerous, but it is easier to start with two people when you are doing it with a horse for the first time. All you really need is two lunge ropes and clip them on to the side rings of a halter or a snaffle bit. Some people use a surcingle and others will run them through the stirupps of the saddle. Of course your horse needs to be used to a saddle first. You can also do this on older horses.

Funny, my friend just told me she wanted to show me how to do this the other day. We didn't have much time to talk about it, and I can see the benefits for ground driving for a young horse, but are there benefits for an older horse? I know what I'm looking for when longing, but what should I look for when ground driving?

I had a gelding that had a stifle injury and I would ground drive him because longing would put to much stress on the injured leg. If you need controlled exercise then you can work in straight lines and ground drive. When we warm up our horses we will ground drive them sometimes instead of lunging them. You can do everything you would do when you are riding. You want to be behind them but off to the side a bit so they can see you. Being behind the girth line, you are driving them forward so they should work as if you were riding them. Some horses will want to turn towards you and if they do that just do your best to stay behind them and work your reins to get them going forward. You can use this to train them on different bits, especially if you don't know how the horse will react. You can teach them to turn towards the rail and turn on their haunches. There are a lot of uses and I use it pretty often.