This, I know something about. I've done off-grid power installations including for RVs, and this isn't that different.
If you don't spend a lot of money and it just says "inverter" it'll be basically "square wave". AC power shifts polarity (basically "jumps back and forth") 60 times per second in the US, 50 in Europe and some other places. Each time is shifts, in a square wave it does so "harshly". This is actually OK for any electronics that run off a "wall wart" or "power brick" adapter that turns AC input into DC output.
BUT if anything uses AC to directly run a motor (microwave oven, power tools, electric tire inflator, stuff like that, crappy power is bad.
Modified sine is generally good enough, like BlakDrgn says. The really cheap inverters you find at Walmart and truck stops are good ONLY for laptops and other fairly light loads. If you can find one in the 250watt range give or take, and want to use it just for a laptop or similar, that's fine.
The other thing you need to know is that inverters that plug into a cigarette lighter socket should not be used for more than 200 watts max. That's not based on the inverter exactly, it's based on the load. If all you have is a 300watt inverter and you use it off a cig lighter, fine, but don't try and run a very potent gaming laptop with much more than 175watts draw tops through it.
How do you figure the draw? Well if you look at a laptop's power brick, it'll tell you the amount of amps and volts it is delivering. Wattage can be calculated from those. For example, my laptop sucks down 19v DC at 3.3amps, which per this page:
http://www.supercircuits.com/resources/tools/volts-watts-amps-converter
...works out to 62.something watts. So for this laptop, a 100watt mini-inverter would cope. I have a 200watt primary and 140watt spare with a slightly noisy fan :).
Last advice on these cheap small inverters: don't run them at more than 75% of their capacity. If you have a laptop that draws 150watts of juice, I wouldn't run it on a 150watt el cheapo Chinese inverter! 200watt, yeah, OK.
If you're buying an inverter bigger than 300watts to drive major loads like a microwave oven (typically 900watts), those bigger inverters need to be "hardwired" into the battery bank. With BIG-ass fat cables and massive fuses from hell, plus properly insulated so they don't short out at key points and burn the truck to the frame. PROFESSIONAL INSTALLATION ONLY unless you truly know WTF you're doing. This is why all the big trucking companies say "don't install your own inverter" - they're talking about the hardwired types, not the little mini-inverters for cig lighters that really should stay at 200watt or under.