If you google "hand sewing needle types" - you will get lots of places to look
Betweens are shorter in length than sharps. Betweens have a round eye. I don't remember what shape the eye is in a sharp. I know that embroidery needles have elongated eyes.
A finer (thinner) needle is easier to push through the layers (as long as it doesn't bend).
The numbering system of hand needles: The larger the number, the thinner and smaller the needle.