The league is your most important competition. During a season you'll play at least twice against every other team in your league. In small leagues you'll even play 3 or 4 times against each opponent. The number of teams in your league corresponds to the number of teams in the real league of your country (england has 20, germany 18 etc).
Leagues are divided in levels. Lower levels also have multiple subgroups on the same level.
Level 1 and 2 have only one group, after that the mount of subgroups is doubled each level. Level 3 has 2 subgroups, level 4 has 4 etc.
New levels are added automatically when there are only a few free teams left in your country.
The number one at the end of the season automatically promotes to the next level. The two lowest teams automatically move down one level.
There will also be teams participating in playoff matches at the end of the season. The amount of playoff spots depends on the number of teams in a league and the level you are in. You can see which spots go to the playoffs by the red 'down' and the green 'up' icons in the league overview.
The league playoffs consist of two home-and-away knockout rounds. In the first round, the teams from the lower levels battle each other. The winners over two matches then advance to the second round where they'll meet a team from the higher level. The winners of the second round move (or stay in) the higher level, the losers move (or stay in) the lower level.
From season 5 onwards, league playoff matches don't give any stadium income or popularity. You also can't sell tickets to playoff matches.
Teams participating in the playoffs can be moved to a different subgroup even if they stay in the same level. A team failing to promote from 3.2 to 2 might play the next year in group 3.1, but it might also stay in 3.2
Teams that didn't qualify for playoffs will stay in the same subgroup for the next season.
As the size of the league differs between countries, the number of league matches you play in a season also differs. To compensate for this, the match income from league matches is higher in leagues with less matches. You also get more fans per match and from ticket sales.
This means the income from league matches can be higher than for cup matches. You can see how much you earn for each match in the stadium.
In leagues where you play each opponent twice, you will play each opponent once at home and once away.
In leagues where you play each opponent three times, the third match will be played without home advantage. One side will still collect the stadium income but they will not have an advantage in the match.
In leagues where you play each opponent four times, you will play each opponent twice at home and twice away.
Each player can get experience in his first 20 league matches each season. For each of these matches he'll get 500 experience points.