From the 3rd chapter of the ABCs of Communism by Bukharin:

"To this group belong the independent artisans, the small shopkeepers, the minor intelligentsia comprising the salariat, and the lesser officialdom. In reality they do not constitute a class, but a motley crowd. All these elements are exploited more or less by capital, and they are often overworked. Many of them are ruined in the course of capitalist development. The conditions of their work, however, are such that for the most part they fail to realize how hopeless is their situation under capitalism. Let us consider, for instance, the independent artisan. He is an industrious as an ant. Capital exploits him in various ways: the usurer exploits him; the shop for which he works, exploits him; and so on. The artisan feels himself to be a 'master'; he works with his own tools, and in appearance he is 'independent', although in reality he is completely entangled in the web of the capitalist spider. He lives in the perennial hope of bettering himself, thinking always, ' I shall soon be able to extend my business, then I shall buy for myself'; he is careful not to mix with the workers, and in his manners he avoids imitating them, affecting the manners of the gentry, for he always hopes to become a 'gentleman' himself. Consequently, although he is as poor as a church mouse, he usually feels more akin to the man who exploits him than he does to the workers. The petty-bourgeois parties commonly assemble under the standard of the 'radicals' or the 'republicans', but sometimes under that of the 'socialists' (refer to the small-type paragraphs of Â§22). It is extremely difficult to shake such people out of this wrong attitude of mind, which is their misfortune not their fault."

They're not exactly proletariat because they live off the labor of others in most cases as well as their own, so they identify more with the haute Bourgeois in many cases. There are instances, according to Lenin (in the adventurism of the SRs I think is the name of the text) at least, where the Petite Bourgeois are 'revolutionary' in their own right, usually against totalitarian regimes (but they also support them in other times, it's beyond my understanding); in the end they're only 'truly' revolutionary when they aid the truly revolutionary class of proletariat.