The field of a shield in heraldry can be divided into more than one tincture, as can the various chargess. The divisions are named according to the ordinary that shares their shape.
Common partitions of the field are:
A shield vertically divided into blue (left side) and gold (right side) would be blazoned: Per pale azure and Or.
The arms of the former Republic of Bophuthatswana were "per fess (at nombril point)" (lower than the regular per fess division; the "nombril point" is halfway between the fess point -- the exact middle of the field -- and the base point, at the bottom centre of the field).
The arms of the State of Wisconsin show a quartered shield in which each of the four quarters has a field of or, but this is of questionable propriety.
The arms of Mpumalanga Province in South Africa show "per bend sinister, inclined in the flanks per fess."
There can also be party per chevron reversed, which is like party per chevron except upside down. Party per chevron reversed throughout (with the point reaching to the very bottom of the shield) is sometimes referred to as chaussee.
Shields may also be divided into three parts: this is called tierced, as in tierced per pale, azure, argent, and gules (though in British heraldry this is not done and the foregoing shield would be blazoned [as the pale is supposed to be one-third of the width of the field and is always so depicted under these circumstances] per pale azure and gules, a pale argent.) A particular type of tiercing, resembling a Y in shape (division lines per bend and bend sinister coming down from the chief, meeting at the fess point, and continuing down per pale), is called per pall. (The arms of Sine ni Shranachain from the Society for Creative Anarchronism Barony of the Forgotten Sea show this, but are inaccurately blazoned "Per pale argent and Or, on a chief triangular purpure a cat's face Or".)
A field pily, as in the arms of the 2nd Baron Marks of Broughton, Michael Marks, is similar to a field per fess dancetty, except that the teeth are much more exaggerated.
The division line may be of any of the different line shapes.
One division of the field (though it is described by some as a charge) is restricted to the chief: when the chief is divided by a bow-shaped line, this is called chaperonnet.
See also variations of the field.
Reference: Burke's Peerage and Baronetage, 105th edition.