Steve Camp points us to an excellent essay by John Hendryx on the critical distinctions between Monergism and Synergism. Hendryx writes:

Monergism: The doctrine that the Holy Spirit is the only efficient agent in regeneration – that the human will possesses no inclination to holiness until regenerated, and therefore cannot cooperate in regeneration. Monergism is when God conveys that power into the fallen soul whereby the person who is to be saved is enabled to receive the offer of redemption. It refers to the first step (regeneration) which has causal priority over, and gives rise to, the spiritual ability to comply with all the other aspects of the process of being united to Christ, (i.e., the ability to apprehend the Redeemer by a living faith, to repent of sin and to love God and the Mediator supremely) It does not refer to the whole process that it gives rise to (justification, sanctification), but only the granting of the spiritual capacity to comply with the terms of the covenant of grace.

Synergism: “…the doctrine that there are two efficient agents in regeneration, namely the human will and the divine Spirit, which, in the strict sense of the term, cooperate. This theory accordingly holds that the soul has not lost in the fall all inclination toward holiness, nor all power to seek for it under the influence of ordinary motives.” This unscriptural view is the greatest threat to a true understanding of salvation in the Church today.

The essay goes on to point out how these differences affect our view of regeneration, humanity, and the gospel. I commend it to you and thank Steve Camp for pointing us to it.