Hi Carolyn.
Don't worry about being confused, it's part of learning and it's good you ask questions.
The thing with identifying glass is that you have to take into account a multitude of factors. Mere shape is never conclusive and as Cathy said, other factors include weight, colour, the finishing of the base or rim, tool-marks, any additions to the glass - such as metal inclusions and over-all 'feel'.
It's only once you've ticked all the boxes that you can make a stab at attribution for an unsigned piece. Of course, more experienced 'glassies' can get a 'gut feel' about a piece from fewer certainties, but that takes time and a bit of 'sticking your neck out'. :lol:
Keep asking questions, we're all still learning here (speaking for myself really!) - it never stops, and you'll pick up information as you go along.