How does Karl Marx differentiate between members of the proletariat and the bourgeoisie?

The bourgeoisie is the upper class, people who own and control the means of production, such as factories, etc. The proletariat are people who have blue collar types of jobs, in mining, manufacturing, farming, etc., the people who do manual labor. There is no overlap between these two categories because the distinction is not one of wealth but of function within the capitalistic economy. The bourgeoisie buy the labor of the proletariat and oppress them by squeezing as much value out of them as possible in order to accumulate wealth, but they do not have to be wealthy to be a member of the bourgeoisie. The proletariat own nothing but the right to sell their labors.

Marx did see a middle class, which he called "the petty bourgeoisie." These would be lawyers, doctors, architects, small shop owners, etc. They do not own or control the means of production but they aspire to be wealthy, therefore they are more like the bourgeoisie than the proletariat.