So first things first, you want to learn about lighting modifiers, it helps to understand the difference between hard and soft light. What do these terms mean? Well, the softness of the light tends to affect the transition from highlight to shadow, if it's a very abrupt change, then you have yourself hard light, if it's a gradual change, well then you have soft light. What does this actually look like, well you can see below.
Hard Light vs. Soft Light
So you can see in the hard light, we have the shadow behind the subject and the shadow is very abrupt and obvious whereas for soft light, the shadow is not as defined so it's harder to say where exactly the shadow begins. Generally, for portraiture people like using soft light as this tends to be more flattering. The question may be then well how do you achieve this. Well, the softness of a light is the product of two different relationships:
The relative size of your light source compared to your subject
The distance between your light source and your subject
So basically, what this means is that the softest light is made by large, close lights. A large, close light will give you softer light than a small, far-away light. So, for the images above, the one on the left is done using the bare head of my speed light whereas the one of the right is made by using a shoot through umbrella. By using a shoot through umbrella, we make the light source a lot bigger which makes the light a lot softer. I could bring my umbrella closer to make it even more softer or buy a larger umbrella. Conversely, if I moved it far away from my, suddenly my light would not look so soft and the transition between highlight and shadow would be a lot more abrupt.
So, now that I've mentioned the basics of how you can get soft light, I'll talk about the two major modifiers you can use to get it, soft boxes and umbrellas. I personally prefer white umbrella to black umbrellas.
White umbrellas are shoot through umbrellas which means you face it towards your subjects and fire the light through the umbrella. Black umbrellas have a silver lining which reflects the light so you face the umbrella away from your subject so that the light bounces on the inside of the umbrella and then comes back towards them. With soft boxes you generally would face them towards your subject since all their light comes out the front.
The curve of the umbrellas mean that they give a little more fill as they scatter light everywhere. Softboxes on the other hand tend to be a little more directional so you might get a little more shadows on your subject.To be perfectly honest, I find they look fairly similar but there is a slight difference which I've tried to exaggerate a little in the photo below:
Umbrella vs. Softbox
As you can see, the umbrella has scattered light more so has a little more fill in the shadows than the softbox. The bigger the umbrella/soft box, the softer your light. That's the basic thing to take away from this. Also, I have kept the flash in the same position for all of the above photos so you can get a better comparison. I have left the flash power relatively constant although you often need to adjust it because certain modifiers can cause a loss of lights so you may need to turn your flash power up or down in different situations.
There are many other modifiers which can be interesting to use so I'll mention some here. The first is a beauty dish which is known to give semi-hard light. A beauty dish tends to focus your light a lot too so you might need to turn down your flash power or you'll blow out your highlights.This is what a beauty dish looks like if you aren't familiar with them.
What a beauty dish looks like
Here is a comparison shot for the beauty dish and the bare bulb, you should be able to see the shadow is not quite as defined as the bare bulb, but not as gradual as the softbox either which is why we say semi-hard light.
Bare Bulb vs. Beauty Dish
So, what other modifers exist? Well, some of my favourites are snoots and grids. Both of these tend to focus our light to give a spotlight effect but the snoot tends to give an abrupt edge whereas the grid tends to give a gradual transition to darkness. Here is a comparison of the grid and a snoot so you can see what I mean. Both tend to give fairly hard light if you're using them with a speedlight. However, you can buy grids for giant softboxes which can give you very focused light while being fairly soft, so keep this in mind.
Grid vs. Snoot
And just so you know, here is what a snoot and a grid actually look like. My grid is a bit battered and I don't actually own a snoot, I just roll up a piece of black cardboard and stick that on my flash, it works surprisingly well.
What a grid and a snoot look like
And just in case you were curious, I also decided to take a shot without a snoot or a grid so you can get a direct comparison of what it actually does, check it out:
Bare Bulb vs. a Snoot
So there we go, that covers most of the basic flash modifiers that are out there. The other big one is gels I suppose but I feel I have covered that many times before in other posts so I won't do it again here. Anyway, as you can see flash modifiers can give you some awesome effects so be sure to add them to your collection if you haven't al ready. If you can't be bothered purchasing them, there's plenty of tutorials on line as to how you can make your own modifiers from cheap things like baking trays and straws. So enjoy and happy shooting.