The X-axis is the line on the bar graph running along the bottom. This usually charts the non-variable data of the graph, such as a person's name or a country. Sometimes the X-axis runs up the side of the graph, which is usually the Y-axis, as the creator wants the bars to run horizontally instead of vertically. This is just personal preference.
The Y-axis runs up the side of the graph and usually plots some variable data. This can be to measure the monetary value of countries' exports or the grades a class is getting in exams. Like the X-axis, it is possible to run the Y-axis described here on the horizontal; it is just a personal preference.
The bars running up the graph now represent the levels and make it easy to distinguish between the different groups and make measuring the results easier. If there are several non-variables on the X-axis, it is a good idea to make them different colors to make identifying them easier.
It is common to find bar graphs with two colors on each bar. This is so the non-variables on the X-axis can be broken down into a subgroup. For example, if you were making a graph which had school classes on the X-axis and average grades on the Y-axis, two-toning the bars would allow you to split each class into boys and girls.