This is a bit of a rerun. What follows is the modified answer to a Doctrinal Quiz Show from back in February (omitting the fun stuff like the bonus question and Bubble Awards!). It's a good prelude to my next (half-written) post that, God willing, will be fully written before you all forget there ever was such a thing as a Little Catholic Bubble! Okay, here we go….
There are two kinds of grace: sanctifying grace and actual grace.
Sanctifying grace is the very life of God (let that sink in!). It is interior grace, which dwells in the soul and transforms it, making it holy. The ordinary way we receive sanctifying grace is through the sacraments.
Actual grace is supernatural help from God (like nudges). It is external grace, working on us from the outside only; it does nothing to change the state of our soul. Everyone receives actual grace, and it comes to us when we need it. We have the free will to cooperate with it or ignore it.
Let's go into a little detail about each.*
I'll start with an analogy that was very helpful to me: We all know that our human nature is not equipped to live in the ocean. We will die there unless we have something outside of (or above) our nature to sustain us. Scuba gear is the apparatus that allows us to transcend our human nature and live underwater. Similarly, our human nature is not equipped to live in Heaven. Sanctifying grace is the "apparatus" that allows us to transcend our human nature and live in Heaven. We need sanctifying grace, for in our merely human state, we are simply not equipped to live in the presence of God.
When God created our first parents, Adam and Eve, He immediately infused them with the extraordinary gift of sanctifying grace (i.e., a sharing of His divine life). This undeserved gift elevated them above their own human nature and allowed them to live in the direct presence of God. Unfortunately, Adam and Eve threw away this supernatural gift when they "fell from grace" by choosing to sin. Without sanctifying grace, they were now separated from God, and Heaven was closed to them.
Since the Fall, humans are no longer given the gift of sanctifying grace at their creation. However, thanks to Jesus Christ and His atoning death, we are able to receive the gift of sanctifying grace through the sacraments of His Church. At our baptism, our souls are infused with sanctifying grace (God's very life!), and we become fit for Heaven. Assuming we continue to live in God's friendship and not commit mortal sin, this grace can continue to grow in our souls throughout our lives (through our prayers, acts of mercy and love, penance, etc.), making us increasingly holy. We hope to die in a state of grace.
Actual grace is like little nudges from God. These nudges can take many different forms and come to us from many different places (and people). Actual grace could come in the form of your mother reminding you not to miss Mass on Sunday, or a friend telling you that you should get to confession. It could be a catastrophic event which leaves you on your knees before God, when before you wouldn't have gone near Him. It could be the promptings of your own conscience when confronted with your sins. It might be a class you took or a book you read which compelled you in the direction of justice or righteousness or humility. It could be the unexpected presence of a holy person or the feeling of awe you get when you look at a coastal sunset. It could be just about anything which moves you closer to the Lord.
Actual grace can be acted on or it can be ignored; but just like a train, there will always be another "nudge" coming around soon. God never abandons anyone, and every human being on the planet receives as much actual grace as is necessary to prompt one to seek out salvation. Unlike sanctifying grace, actual grace does not touch the interior of a person, but only works from the outside. Actual grace does not change the state of a person's soul, but is merely the agent that pushes a soul to seek out and receive sanctifying grace.
*When I taught myself the Catholic Faith in the mid-90s (before I had internet), I bought a great set of little tracts from Catholic Answers. They were chock-full of practical, easy-to-understand information, and much of what you are reading here is from those tracts. You can find all of that today on their website.