Mixed voice is basically a mix of your chest voice and your head voice (not falsetto, although i think there is a technique called "reinforced falsetto" which sounds quite similar)
I'm not a professional, but I think Kitty and Monica were right about it being in the middle of your chest voice and head voice, just at your vocal break (where it cracks or flips). The vocal break varies in people's voices. For some it's A3, maybe C or C#4, maybe E or F or F#4, it's just not the same for everyone.
The mix can also be used as a healthier alternative to belting. Belting is mostly just shouting and pulling chest voice. With the mix, you can have the power and loudness of your chest voice but without the strain, and the ease and lightness of head voice.
You can't achieve that with falsetto. Falsetto has a very breathy sound because the vocal cords aren't coming together, which allows a very large amount of air flow through the cords and gives it that weak, breathy sound. But of course the falsetto is very good for effects and beautiful soft notes.
You need to be able to use your chest and head voice (once again, not falsetto) to use your mix.
There is a video that explains the difference between head voice and falsetto. If I'm remember correctly, it's called "Falsetto vs Head Voice Adduction" by RockTheStageNYC on YouTube.
There are exercises for strengthening the mix. Try lowering your soft palate and sing a note just above your "flip" or "crack" in your head voice very softly and increase volume slowly until you get a big and powerful sound, but without straining or forcing your vocal cords.
Or you can check out SingingSuccess.com or SingingSuccessOnline.com. They've recently released a new program called "Mastering Mix".
I don't have it yet, but I'm pretty sure it would be helpful.
I think there is also a mix of chest and falsetto just like Cockney_rebel79 said, but it sounds breathy and soft, unlike the mix of chest and head.
Check out Singing Success, Kevin Richards, Rob Lunte, and some other cool vocal coaches on YouTube.
Once again, I'm not a pro so my answer maybe isn't that precise, but you get the idea :)