You're talking about ND GRADUATED filters if you mention hard and soft - the OP is after ND filters that are the same density across the whole area. Quite different and used for different purposes.
To the OP - first thing is to understand densities - how dark the filter is. 1 stop cuts the light by a half, 2 stops to a quarter of the original amount, 3 stops to an eighth and so on. You will also see them as 0.3, 0.6 and 0.9 - this is equivalent to 1 stop, 2 stops and 3 stops respectively. You can get them darker as well - the "Big Stopper" 10 stop filters that will reduce it to around 1,000th of the amount - even more.
Why would you use them? Firstly to give you a longer shutter speed than you would otherwise have had or, often neglected, to allow you to open up the aperture to help reduce depth of field more than you would otherwise have been able to do.
General rule of thumb is avoid the cheap variable ND filters - they can leave all kind of unwanted artefacts on the image - they're effectively two polarisers - so might give you those effects when you don't want them.
Best advice - buy a low cost set - but not the cheapest - say £10-£15 off e-bay. See what you use most often and then look at a more expensive specific option from the likes of B&W, Lee and so forth.
) but the point would be to use them to learn what you want to get and how you want to use them and then you can buy a decent set. Maybe a square filter/holder system.
