Each War Star collected during a Season counts towards a Clan's War Star amount, which in turn factors into whether the Clan is promoted or demoted at the end of War Week. If a Clan wins a Battle Day, they are awarded 10 extra War Stars which helps them get promoted.
At the end of a War Week, Clans that have earned the most stars are promoted to higher leagues while others stay or are demoted. In most Leagues the two Clans with the most stars are promoted and the two Clans with the least stars are demoted. These rules vary by League however.
If Clans are tied on stars, the total destruction percentage serves as the tie-breaker.
You can see Season statistics by tapping the
Season info button in the War Map. The
Group tab shows the current standing of Clans in your Group:
Tapping on a Clan's stars shows a breakdown of where their War Stars come from:
Rounds tab is a quick way to check how what happened in the Wars between other Clans in your Group:
The Clan tab is a great way to keep tabs on how your Clan mates are performing. It is worth noting that the Clan tab only updates once Battle Day has ended: