Simple Heraldry is just that, an extremely straightforward heraldic device. Simple heraldry is difficult to define, as it varies from kingdom to kingdom. Typically, simple heraldry consists of a single device on a plain field, with no divisions. Fur is also not used in simple heraldry. Fieldless devices do not count as simple heraldry.
In some kingdoms, simple heraldry requires only a single point of difference from the nearest arms, rather than two points of differnce.
For example, OR a unicorn azure is simple heraldry, and although OR a tower azure has only a single point of difference, it would pass the college of heralds and be approved.
Simple heraldry is not all that common in the SCA, as most of the simple designs have already been claimed in the armorial.