A mixture, in chemistry, contains two or more substances that are not chemically bound to each other. You can usually identify the substances in a heterogeneous mixture simply by looking at it: the different components are easy to tell apart from each other. (A bowl of peanut M&Ms would be a good example of a heterogeneous mixture: the nuts and different colors of candy are not distributed evenly, and you could easily separate them if you wanted to.)
In a homogeneous mixture, the substances are distributed evenly and blended so well that you can’t tell one from the other, so the resulting solution looks like a uniform substance. Sea water contains salt and H20, but you can’t see the individual salt crystals, because they’ve dissolved in the water.
It’s important to distinguish mixtures from compounds. In a compound, the substances are chemically bonded, forming entirely new molecules. The H20 part of sea water is actually a compound: each oxygen atom shares electrons with two hydrogen atoms. The salt is dissolved by the water but does not become part of the water molecules.