For most of baseball's history, home-field advantage in the World Series alternated between the American League and National League each year. Following the infamous 7-7 tie in the 2002 All-Star Game, the Midsummer Classic was used as a mechanism to decide home field in the Fall Classic, with the champion of the league that won the All-Star Game getting home-field in the World Series from 2003 through 2016.
• 2018 postseason bracket and schedule
That all changed before the 2017 season, and home-field advantage in the World Series is no longer based on the result of the All-Star Game. Home field (Games 1-2 and 6-7) in this best-of-seven series goes to the team with the higher regular-season winning percentage, regardless of whether team is a Wild Card winner or a Division Series winner. (Of course, in the first World Series in which these rules were implemented, the home team lost Game 7, with the Astros winning 5-1 against the Dodgers in Los Angeles to become 2017 champs.)
If the two pennant winners go into the Fall Classic with identical regular-season records, the first tiebreaker will be head-to-head record in the regular season. If the teams did not play during the season during Interleague Play, the tiebreaker would go to the team with the better overall record against teams in its own division. If that were also a tie, the tiebreaker would be intraleague records (the NL club's record vs. NL teams and the AL team's record vs. AL teams).
Below are the World Series home-field advantage seedings for 2018, which are based on the regular 162-game schedule and do not factor in the two tiebreaker games played in the NL (aka Game 163). And while the Cubs rank higher than the Brewers here despite finishing second to Milwaukee in the NL Central after losing Game 163, it's a moot point because they cannot meet in the World Series. In the DS and LCS, a Wild Card team cannot have home-field advantage, even if they had a better record during the regular season.
Home-field hierarchy for 2018 World Series

1. Red Sox (108-54)
   
2. Astros (103-59)
   
3. Yankees (100-62)
   
4. A's (97-65)
   
5. Cubs (95-68)*
   
6. Brewers (96-67)
   
7. Indians (91-71) **
   
8. Dodgers (92-71) ***
   
9. Rockies (91-72)
   
10. Braves (90-72)
    * CHC won season series vs. MIL, 11-8
    

** Did not play COL or LAD, but had better intradivision record
*** LAD won season series vs. COL, 12-7