Let's take the first part, the IRS employees. When a group files for tax-exempt status, the IRS investigates it, asks it some questions, and determines whether it qualifies under section 501(c)(3) or 501(c)(4). The difference between them is that a 501(c)(3) is supposed to be a genuine charity, like your local food bank or Institute for the Study of Foot Fungus, while a 501(c)(4) is still primarily devoted to "social welfare" but is allowed more leeway to engage in some political activities like lobbying and participation in elections, so long as the political activities make up a minority of its time. The biggest practical difference is that donations to (c)(3) groups are tax-deductible, while donations to (c)(4) groups are not.
But:
The truth is that a great many of the groups that request 501(c)(3) and 501(c)(4) status, of all ideological stripes, are basically pulling a scam on the taxpayers. Maybe that's a bit harsh, but at the very least they're engaged in a charade in which they pretend to be "nonpartisan" when in fact they are very, very partisan. For instance, nobody actually believes that groups like the Center for American Progress on the left or the Heritage Foundation on the right aren't partisan. When there's an election coming, they mobilize substantial resources to influence it. They blog about how the other's side's candidate is a jerk, they issue reports on how his plans will destroy America, and they do all sorts of things whose unambiguous intent is to make the election come out the way they want it to. CAP and Heritage, along with many other organizations like them, are 501(c)(3) charities, meaning as long as they never issue a formal endorsement and are careful to avoid any express advocacy, they can maintain the fiction that they're nonpartisan (keep getting tax-deductible contributions, which are easier to obtain than those that aren't tax-deductible).
