Field Spell Cards have the advantage of being able to change the entire state of play, for both you and your opponents side of the field. Most cards center around boosting ATK, DEF or both, for cards with Specific Attributes, or Types. They are Spell Speed 1.
Field Spell Cards must be placed on the Field Spell Card Zone, and there can only be one in play at any one time. If one is already active, and another one is activated, the previous Field Spell Card is destroyed. You may Set a Field Spell Card at any time, as long as one is not present at all on your Field Spell Card Zone. If you Set a Field Spell Card, and your opponent already has one active, the opponent's Field Spell Card is neither destroyed nor negated.
The effects of Field Spell Cards do not start a chain.
Originally in the TCG, you could not activate another Field Spell Card if you already controlled one, but this rule has since been changed to reflect the process of the OCG; you merely send the Field Spell Card you already control to the Graveyard, and play your new one immediately.