Competitive League of Legends will be getting a new Mid-Season Invitational (MSI) event which pits teams from the five major regions against one another. Teams from elsewhere in the world will be getting a separate multi-region event in which to duke it out.
The Mid-Season Invitational will take place in the period between each global region's spring and summer split. If you're not familiar with the terminology, splits are periods of weeks over which a number of matches are played between regional teams. It's kind of like a football season and there are similar opportunities for promotion into the league from the Challenger series, relegation and player transfers.
The invitees to the MSI will be the winners of the North American LCS, the European LCS, LCK (formerly OGN), LPL and LMS. In non-acronym speak that just means the best teams from North America, Europe, Korea, China and Taiwan/Hong Kong/Macao. There will also be a wild card option which is determined by the aforementioned other multi-region event.
The International Wild Card Invitational (IWCI) will take teams from Turkey, CIS, Oceania, Japan, Brazil, Latin America and GPL. It's a bit earlier than the MSI because this is how you get tha sixth wild card option. The top team from this event will be the sixth participant in the MSI.
Riot have previously held All-Star events at this point on the calendar – last year was Paris, the year before was Shanghai. They too had the top teams from each region facing off against each other but, as you might remember from the Paris event, there was also the All-Star Challenge which involved mixed roster teams competing in novelty game modes or skill challenges.
As Riot said in their pre-Worlds Q&A they'll be looking to hold a more light-hearted event later in the year, so basically it feels like they've separated out the All-Star Invitational from the Challenge. The Challenge elements will resurface in the All-Star 2015 which will take place after Worlds during the off-season in December. The overall impression is that January til Worlds will be SERIOUS GAMEFACE BUSINESS TIME while the off-season is for larking about and letting off steam.