How would you describe calculus in simple terms? I keep hearing about this weird type of math called calculus. I only have experience with geometry and algebra. Can you try to explain what it is to me?

Calculus is basically a way of calculating rates of changes (similar to slopes, but called derivatives in calculus), and areas, volumes, and surface areas (for starters).
It's easy to calculate these kinds of things with algebra and geometry if the shapes you're interested in are simple.  For example, if you have a straight line you can calculate the slope easily.  But if you want to know the slope at an arbitrary point (any random point) on the graph of some function like x-squared or some other polynomial, then you would need to use calculus.  In this case, calculus gives you a way of "zooming in" on the point you're interested in to find the slope EXACTLY at that point.  This is called a derivative.
If you have a cube or a sphere, you can calculate the volume and surface area easily.  If you have an odd shape, you need to use calculus.  You use calculus to make an infinite number of really small slices of the object you're interested in, determine the sizes of the slices, and then add all those sizes up.  This process is called integration.  It turns out that integration is the reverse of derivation (finding a derivative).
In summary, calculus is a tool that lets you do calculations with complicated curves, shapes, etc., that you would normally not be able to do with just algebra and geometry.
