In his "Second Way" Aquinas argues from efficient causality. So what's that?
Well, imagine a violin. Thinking in terms of Aristotle's "Four Causes," Aquinas would say that the (1) "material cause" of the violin is the stuff it's made of --- the wood and wires and so forth. The (2) "formal cause" is the form or pattern, the structure the violin exhibits. The (3) "final cause" is the end, the goal, the purpose of the violin. The (4) "efficient cause" is that which actualizes the potential of the wood and wire and brings the violin into being --- the violin maker and his tools.
Taken together these "four causes" provide a complete "explanation" of the violin. In order for a particular violin to come into being someone had to have the goal in mind of making a violin. He had to have the form or pattern or blueprint. He had to have the material. And he had to have someone who could use the pattern and material and bring the thing into being. This is the violin's "efficient cause." It's what we usually have in mind when we use the word "cause."
Now, with this in mind, let me simply state the argument.
1. Our senses reveal to us an order of efficient causes in the world.
2. Nothing can be the efficient cause of itself because then it would have to exist prior to itself, which is impossible.
3. In a series of efficient causes, each member of the series is the cause of the next.
4. Because of this, if there is no first cause in the series, there will be no series at all.
5. The series of efficient causes cannot extend infinitely into the past, for then there would be no first cause and therefore no series.
6. Therefore it is necessary to admit a first efficient cause, to which everyone gives the name of God.
Now, let's explore this in more depth.