Season tickets typically guarantee you access to all games with the exception of cup games. There are packages of season tickets that come with an assigned seat. Some cheaper packages may include standing option only.
Cup games are not included because there is no way for a team to know beforehand how many games there will be in a cup. For example, if it turns out that your team goes all the way to the finals and you were to get the tickets to all the games at the beginning of a season, this could be a very expensive season for your team.
Season tickets have a number benefits compared to tickets to separate games. Not only do season tickets guarantee you seating, they are also cheaper than tickets for individual matches. However, the disadvantage of season tickets is that you have to pre-pay for a lot of tickets, and this may get expensive. Depending on the club, prices for season tickets start at about £200, and can reach thousands and even tens of thousands for the most popular clubs in the country. Often, you can buy combined season tickets. When you have such tickets, you can take your children with you to the games. Popular clubs may also have a waiting list of several years for season tickets.
Often, season tickets holders have certain benefits and extra rights. For example, teams often offer them first option on tickets for away games, which are typically in very limited supply. Should the team get into finals of a major cup, season ticket holders often get booking privileges for those games, too.
For all these reasons you want to keep an eye on when your favourite team releases its season tickets. Often, there is a period when only current season ticket holders can get tickets for a new season.