There's a wide range of semipermanent products on the market, and for good reason: they're relatively easy to use at home (though salons use them too), they don't require a major commitment and they're a good bit less damaging than permanent colors.
Semipermanents run the gamut from almost temporary to almost permanent: Some deliver very little color inside the hair shaft, depositing it mostly on the cuticle, where it lasts as few as two weeks. Others send more dye inside to the cortex, where it will remain until the hair grows out. And some do contain peroxide -- you can't lighten without peroxide -- but usually in fairly low concentrations. Those that are at the more permanent end of the scale also contain ammonia or a similar ingredient.
These products can only alter hair color by about two shades, making dyeing your hair a less scary proposition (and your roots won't look too bad, either). And while they can enrich or highlight your natural color or mask small amounts of gray (up to 50 percent with the strongest products), semipermanents can't completely cover large amounts of gray -- for that you need to use a permanent color.