The WEC is an 8 round championship of 7x 6 hour races at various tracks, and the Le Mans 24 Hours. The 24 Hours comes with double points for the WEC due to its length.
Then, as its such an epic race and has capacity for a bigger field, the ACO invite teams from the ELMS and the TUSC (excluding Daytona Prototypes) based on their performance and results.
Only WEC teams score WEC points. This causes a bit of controversy. Last year only 1 P2 car that competed at Le Man's was a WEC car. They came something like 14th in class yet still earnt the full double points for the 'win' at Le Mans, and then went on to won the championship.
If you want to follow sports car racing you have lots to watch theres ELMS, TUSC, I think there's an Asia series now, and then for GT theres a plethora of top quality GT series across the globe. The P1 Hybrids only compete in WEC however as that's the only series with the P1 class.