Block programming involves scheduling a series of related shows which are likely to attract and hold a given audience for a long period of time.
Reruns on cable television are often assembled into similar blocks to fill several hours of generally little-watched daytime periods.
Generally speaking, block programming is anathema to modern competitive commercial radio, which traditionally uses uniform formats, other than a handful of speciality shows in off-peak hours such as weekends (for instance, the infamous beaver hours in Canadian radio).
The case of talk radio is indicative of the decline of block programming: prior to the 1980s, it was not uncommon to mix various blocks of talk programming together on one station, but this has declined dramatically in the late 1990s and beyond.
Block programming of this nature is alive and well on outlets like public radio (such as NPR, the BBC, or CBC) and in multicultural radio serving broad ethnic and cultural audiences, although even in this realm the idea of block programming is declining due to competition for donations.