What's the difference between MSI and Worlds?

worlds is end of season and crowns the end of season champion for the year. MSI is the mid season equivalent but has a smaller prize pool and has slightly less importance.

MSI invites fewer teams (1 from each regions) and mostly rewards the top 4 teams with a first seed for world's draft. I believe the format is also fairly different, though I haven't checked that in details.

MSI is pretty much the Spring Split international event. Ignoring the inner details, it's basically the best from each of the 13 regions gathering to compete for title and recognition into the upcoming Summer Split. In the past, MSI results would deliver an opinion of expectation for the respective regions going into Worlds. As of 2016, the results would also produce seeding based on the top finishers.

Worlds is pretty self explanatory: the big one; sometimes the only tournament that matters. Once again ignoring inner details, instead of the best from each region, the 5 major regions consist of the best 3 performing teams both present and on the performing year as a whole while the 8 minor/wildcard regions retain choosing the sole best at the time (plus 1 for a select region based on MSI result). How this translates into next year will be known once that time comes.

The top team from each region isn't necessarily the one that performs best internationally. Having more teams from most regions at worlds makes for a more interesting competition in many ways and better showcases each region with multiple teams in the event. MSI - at least this season - really shows off how far "wildcard"/up-and-coming regions have progressed. It also affects seeding for worlds. If you are watching MSI, I'd say the first two stages are probably the best/most exciting - getting to see teams you probably don't normally watch and see them make plays that match the best of the major regions as well as others that make you think you aren't all that bad of a shotcaller despite being a million elo lower than them. Rest of MSI is probably not going to be overly exciting as compared to the first 2 rounds we've had.