Solitaire games are a group of 1-player card games as diverse as they are alike. They are a family of games designed for one player only (thus the name Solitaire), as a fun way to pass time while still stimulating the mind by drawing strategies and struggling to keep focused all the time. Interestingly, these games are also referred to as Patience in many countries precisely because players need to be patient and maintain their concentration in order to win.
The gameplay and rules may vary to a greater or lesser extent between the different Solitaire games, but they all have something in common - the players are always expected to rearrange the cards in a certain order.
In popular Solitaire games such as Klondike, Spider, or FreeCell this rearrangement is done in foundations, where the players must organize the cards following an ascending or descending order and by suit. Others such as Tripeaks, Golf, or Pyramid require the players to organize them by rank, with the suit being a constraint or not.
In fact, the constraints of each game are what make them unique and more or less challenging. The rules to build the foundations or to remove cards from the tableau are already a challenging feature. However, the level of difficulty of a game is generally associated with the rules governing the way players can move the cards around the tableau. The existence of a stock pile and the limitations to its use are used to adjust the level of difficulty too.