UEFA Champions League changed its format completely last year, with the debut of a League Phase instead of Group stage, that allowed more teams to participate (36 teams in total), increased the number of matches (eight during the league phase instead of six) and even added an extra play-off round for teams that ended the league phase between 9 and 24.
The format remains the same this year except for one key thing, that has been modified after some clubs, particularly FC Barcelona and Arsenal complained. This change will benefit the best teams in the league phase that finish between 1-8, as they will not only omit the extra play-off, but, here's the change, they will be seeded for the rest of the tournament, meaning they will always play the second leg at home.
Previously, teams that finished 1-8 were awarded home advantage only during round of 16. After that, it was a random draw that decided which teams played their second legs at home or not. Now, the teams that finished 1-4 in the league phase will always play the return leg at home in quarter-finals, and the teams ranked 1-2 will play the second leg at home in semi-finals.
Here's also an interesting fact from UEFA's regulations: "If a seeded team is beaten in any round, the team that eliminates them takes over their seeding position in the bracket path". That means that if a club that ranked 4 in the quarter-finals is eliminated, the team that eliminated them, even if it ranked much worse or outside the top 8, "inherits" that seeding position and will earn the right to play the second leg of the semi-final at home.
This change was made after Arsenal, who finished third in the league phase, had to play the second leg of quarter-finals at the Bernabéu against Real Madrid, who didn't make it to the top 8. Arsenal managed to win in the end. However, FC Barcelona didn't get that lucky, as despite being second in the league phase, had to play the second-leg of the semi-finals against Inter Milan away... with the result we all remember.