Digital ad buying has undergone revolution after revolution to become an increasingly optimized, efficient, programmatic marketplace. TV advertising, meanwhile, is a significantly larger market and has largely (until very recently) resisted the same changes. So how are ads bought and sold for TV?
First, the networks come together for a big set of events called upfronts. The networks show off their shows and get commitments from advertisers to run ads against them. The upfronts are generally the most expensive form of TV advertising and are basically the only way advertisers can select single shows to advertise against. Whatever isn't sold upfront is sold in the scatter market closer to the airing of the show. Networks will also intentionally hold back some percent of their inventory in the hopes of selling it for more later in the season. Networks may sell bundles of shows, including remnant spots of some good shows and some less-popular shows. There are various targeting options where you lose precision over the shows you advertising against, including pure daypart and autofill.
Campaigns can target national audiences, regional audiences, or individual markets. Especially for multi-market media buys, because the market is entirely manual - agencies are particularly valuable for TV. Each negotiation with each market is manual, and there is very little pricing transparency. So continuous access to pricing history and long-lasting media-buying relationships add genuine value.
In digital, we have comScore to "measure" our reach. In TV, they have Nielsen. Nielson "measures" who watches what shows, in what markets, to give some sense about the exposed audiences. They have set-top monitoring devices in a certain number of households that use demographic and other data to identify what audiences are watching what. When you buy ad space, you can sometimes buy a guarantee about reaching a certain quantity of a certain audience. The standard for audience is Gross Rating Points (GRPs). This is the measure used to count exposure against a certain audience. For example, if 30% of your target audience watches a show, and has 4 exposures to your ad, you get 120 GRPs for your audience.
In standard linear TV ad buying, the ad is sent to the network or station well ahead of the airing and approved by the relevant standards committees for quality and FTC compliance. This is a huge market, and this didn't begin to do it justice - so consider this a very, very high-level primer.