1. What is the difference between a histogram and a bar graph?

A histogram is used for CONTINUOUS data which usually results from measurements. The bars are joined together unlike in a bar graph which are usually separate.

2. Do the axes on a histogram always have to begin at zero?

No, although if a scale on an axes does not begin at zero it is usual to put a short zig-zag line at the beginning of the scale to indicate this.

3. What is the difference between continuous data and discrete data?

CONTINUOUS data is usually the result of measurement,

DISCRETE data is usually the result of a counting process