Shareware is a marketing method, not a type of software or even strictly just a distribution method.
When software is marketed through normal retail channels, you are forced to pay for the product before you've even seen it. The shareware marketing method lets you try a program before you buy it. Since you've tried the program, you know whether it will meet your needs before you pay for it.
A shareware program is just like a program you find in major stores, catalogs, and other places where software is purchased, except you get to use it, on your own computer, before paying for it.