Are you a homeowner who wants to use natural light inside as much as possible, but you don't want people outside your home to be able to see in? Maintaining privacy is more than a matter of neighborhood etiquette -- it's a safety issue as well. Anyone who can see inside can see what you have that they might want to take.
So, you have to have some visually protective layer that also allows light in. Your two main choices are blinds and sheer privacy curtains; one offers more control over light while the other offers more coverage:
Privacy Curtains: Obscure and Obscuring
Not everyone uses privacy curtains, so if you're one of those people who doesn't quite know what they are, here's a run-down: Sheer privacy curtains are the gauzy, thin curtains that you often see in windows of homes in British TV programs. These are usually white and often have a lacy pattern that is complex enough to obscure the view inside the room.
They fit underneath regular curtains; when you open the regular curtains, the privacy panels let a lot of light in, but anyone trying to look in would get a very blurry picture because the folds of the curtains create a pretty thick barrier. While still available in the United States, privacy curtains are no longer that common in a lot of areas.
The big issue with privacy curtains is that you can't really control the amount of light coming into the room. You can close the main set of curtains partway, of course, but that creates a tunnel effect, with light streaming in, in one thin line. The corners of the room can become a lot darker than you want.
Blinds: Common Control
Blinds let you control the amount of light while preserving the diffuse spread of light that you get through a set of blinds. If you close the vanes a little bit, you don't suddenly make half the room a lot darker than the other half. However, if you rotate the vanes to let in more light, people outside can also see into your home. That means thieves can see if there's any loot they'd like.
Solution: Floor Plans
One way to have the best of both worlds is to use blinds on upper floors and privacy curtains on lower floors. If you're in a one-story building, blinds work well in areas where there isn't a lot of foot traffic, while privacy curtains can be a great choice for street-facing windows. You could also, technically speaking, combine the two, with privacy panels draped over blinds, but those plus regular curtains can become unwieldy.
If you'd like to see how blinds and privacy curtains might affect the amount of light in your home, contact a company that sells vertical and horizontal blinds. Test out blinds in a showroom, for example, when sunlight is streaming into the business. The sales staff there will help you figure out the best possible combination for your house.