Fans
The majority of your income will come from either your sponsors or from your fans (gate receipts).
Attendance at your matches is determined by the number of fans your club has, and the mood they are in.
Fans are particularly fickle - if you are winning games they are happy, but if you are losing their mood will change for the worse.
These fans are your club members, who have paid up as a show of loyalty to your club and will turn out to support you through thick and thin.
The amount of club members you have is a good indicator as to how many supporters you are likely to see turning up to your matches. But even these members will come and go depending on your successes in the middle.