Cropping and resizing are two different procedures.
Cropping is like looking at your address in a standard window envelope, like you’d get a bank statement in. What you see in that window is only part of the whole letter inside. If you take a pair of scissors to that envelopewith the bank statement still insideand cut around that little glassine window, you are cropping off eveything but your address. This is what cropping does in Photoshop.
Resizing is just that: It takes the full image and either reduces or enlarges it, depending on how you set height and width values in Image—»Image Size…
The manual explains the use of both quite well.
I always need to resize because what I do is for screen display, but have have my own way of doing it.
It does totally depends on the image of course.
I take my photos down to 492 X 369 because I add a 100 pixel border around them usually by expanding the canvas size.
Photos are 2048 X 1536 right out of the camera…if I want a certain part of the photo rather than just crop and resize I take the photo down to 1500 X whatever is is, 1000 X whatever it is, or 800 X 600.
I then create a new image that is 492 X 369 and drag my photo into that, and then fiddle with positioning.
Sometimes I do just crop and then resize..like I said…it depends on the image and what I’m trying to do with it.