Is my vision horrible?

i think my vision is 20/100. i am definitely nearsighted. i can't see a lot of things on the board at school, but i can read everything perfectly when it's right in front of me. i'm going to get gles on Saturday, but i was just curious.. what do you think the vision strength of my gles would be? (the number of the lens strength) and what's the difference of a positive number and a negative number?