I had someone ask me this question recently. My short answer is: in popular usage, not a lot.
The terms election and predestination are often used interchangeably, both referring to God’s gracious decree whereby he chooses some for eternal life. In Romans 8:30 Paul speaks of those whom God has predestined, called, justified, and (in the end) glorified. In 8:33 Paul references “the elect,” apparently a synonym for the predestined ones described a few verses earlier.
A sharp distinction between the two words is not warranted from Scripture, but if there is a distinction to be made, predestination is the general term for God’s sovereign ordaining, while election is the specific term for God choosing us in Christ before the foundation of the world. That is, predestination is the broader category of which election is the smaller subset. Calvin defined predestination as “God’s eternal decree, by which he compacted with himself what he willed to become of each man…Therefore, as any man has been created to one or the other of these ends, we speak of him as predestined to life or to death” (Inst. III.xxi.5). For Calvin, predestination encompasses the entire eternal decree. Election and reprobation, then, represent two different aspects of the decree. The Canons of Dort Article 1 makes the same distinctions.
This delineation is not without merit. The “elect” is always a positive designation in Scripture (e.g., Matt. 24:31; Titus 1:1), suggesting that election implies eternal life (though Rom 9:11 may be an exception to this rule). Predestination, on the other hand, can be used more broadly. Herod and Pontius Pilate, along with the Gentiles and people of Israel, did to Jesus what God’s “plan had predestined to take place” (Acts 4:27-28). Indeed, all of our days are written in God’s book before one of them comes to pass (Psalm 139:16 ). Every form of prosperity and affliction comes to us not by chance, but from God’s fatherly hand (Heidelberg Catechism Q/A 27). Or as Augustine put it, “The will of God is the necessity of all things.”
Does this mean we are “predestined” to marry so-and-so or take a certain job? In one sense, looking back at God’s providential care, we can say “Yes, that’s was God’s plan for my life.” And yet this notion of divine superintendence is not meant to undercut personal initiative and responsibility. Everything happens after the counsel of God’s will (Eph. 1:11), but this is no excuse to neglect the use of means, nor is it a reason to think every decision we make is automatically pleasing to God. God’s sovereign unalterable will of decree is not be confused with his violable will of desire.