Verse 14 of Gen. 3 helps us identify one of the two seeds. In that verse, we find God speaking to the serpent. So "your seed" in verse 15 refers to the serpent's seed or Satan (see Rev 20:2) and "her seed" would refer to the seed of the woman or the seed of Abraham (see Gen 22:18: "And in your seed all the nations of the earth shall be blessed, because you have obeyed my voice.")

Although a vague promise, this provided a basis for hope in the eventual reversal of the powers of evil and a victory that should come through as well as to the Adamic family. The seed of the woman reached fulfillment in Christ. In Gal 3:16, 29 we read, "Now the promises were spoken to Abraham and to his seed. He does not say, 'And to seeds,' as referring to many, but rather to one, 'And to your seed' that is, Christ. And if you belong to Christ, then you are Abraham's seed and heirs according to promise." And Romans 16:20 tells us, "And the God of peace will shortly crush Satan under your feet." It is Christ and His glorified Church — the seed of Abraham — that will have this privilege of crushing out evil. 

There is quite a difference between a bruised heel and a fatal blow to the head. The one may be a painful annoyance (a bruised heel) while the other (a blow to the head) is fatal. Thank God for this faint glimmer of hope after the failure of our first parents to pass their test of obedience in Eden which had its devastating effect on the rest of their posterity — sickness, sighing, crying, pain, and death.

The Bible is not merely a collection of moral precepts, wise principles, and words of comfort. It is much more. It is a reasonable and harmonious statement of the causes of evil in the world, its only remedy, and the final results as seen by divine wisdom. Your question touches on one of the most important truths of the Bible. Its opening pages record the creation and fall of man, its closing pages tell of man's recovery from that great fall, and its intervening pages show the successive steps of the plan of God for the accomplishment of this purpose. Simply stated the Bible tells the story of paradise lost to paradise restored.