Non-ratings periods also occur in the States. The description of Australian non-ratings periods is largely accurate, and so won't be repeated.

In general, the ratings period covers approximately 1/4 the time between the end of September (when all the network shows premiere) to the end of May (when they all have the season finales). There are breaks for a few months at a time in the middle, but I've never cared to figure out when they were.

The most interesting thing about non-ratings periods in the States is that they're basicially irrelevant in terms of setting advertisement costs. Obviously, networks have a vested interest in getting the largest possible advertisement revenues, so they run all of their new shows in the ratings periods.

Formerly, this lead to a period of (most notably) Summer doldrums when absolutely nothing new was on television. Similar bouts happened around Christmas and other non-ratings stretches, but they were less severe as some networks have the occasional new show running.

However, this period of doldrums is beginning to change -- many cable stations (such as the SCIFI channel) create original programming. This programming, while often quite good, is usually unable to compete with the big network shows (eg, in the US, Friends, E.R, and Who Wants to be a Millionaire); these cable networks, therefore, often run their programming in non-ratings periods to capture the unfufilled audience.

On a tangent, show seasons are lined up around the Summer non-ratings period. As I mentioned above, a show will tend to premiere in the Autumn and have its season run until Spring, typicially April/May. Since cancelling a show is easiest when the contracts are up for renewal at the end of a season, many shows (all-too-many or not enough, depending on who you ask) fail to return in the fall, especially if their ratings have been sinking.

For a broadcast network, running a show's new episodes during a non-ratings period is usually a sign of deep trouble for the show, like shifting its time slot; networks often do this for soon-to-be-cancelled shows in order to get their airing commitments out of the way, so they can put something higher-rated, like reruns of the top shows, into the timeslot during ratings-periods.