Freecell is played with a standard 52-card deck. At the beginning of play, all cards are dealt to 8 columns (7 cards each in the first four columns and 6 cards each in the other four) facing up.
Besides the columns (which will also be referred to as stacks), there are also four places to hold temporary cards (known as “freecells”) and four foundations, in which it is possible to place cards of the same suit starting from Aces and ending with Kings.
Sequences of cards on the tableau are built by placing cards on top of cards of a higher rank and of a different colour (black or red).
An atomic move consists of any of the following:
Moving one card from the top of a column or from a freecell to the foundation. (which is possible only if its corresponding foundation's value is lower than it by one)
Moving a card from the top of a column to a vacant freecell.
Moving a card from the top of a column or from a freecell to a parent card on top of a column.
Moving a card from the top of a column or a freecell to an empty column.
It is customary and helpful to group the movement of an entire sequence of cards, by moving intermediate cards to freecells or vacant columns. This will be referred to as a sequence move. If a sequence move involves temporarily moving a card to an empty column, it is known as a supermove.
You should probably play a few games of Freecell, in case you did not already, because knowing the rules alone is not enough to have an intuitive feel of it. There are many available Freecell implementations for various systems, and you should have no problems finding one that you can use. (Chances are that it is already installed on your system.)