Seal script, or tenshotai (篆書体) as its name implies, is a writing style predominantly used in seals in modern times.
Translated from here
Translated by Judit Simon
This ancient Chinese calligraphy style evolved from the earliest forms of printing when the characters were carved into stones. Nowadays characters inscribed in this style can mainly be found in seals and their unique imprint gives the finishing touch to calligraphy pieces. Seal script, like other calligraphy styles has its own rules of writing but it also needs to be adjusted to the shape of the seal. (On personal name seals called hanko (判子) or inshō (印章) which are used in Japan instead of signatures we cannot really find seal scripts because this ancient writing style differs greatly from the modern forms of kanji. All in all it can be stated that this early form of calligraphy reflects man's desire to create something beautiful and harmonious.
In Japan there was no such thing as seal script. It arrived in Japan with the seals. Seal script has two forms, great seal script and small seal script, the former being the more ancient one of the two. Let's take a look at some examples of modern characters and seal- script characters. Seal scripts from left to right: dragon (龍), mouth (口), man in two styles (人); heart (心), big (大), calmness (安), writing (書) and nothing (無) in great seal script and finally horse (馬) in small seal script.
For lovers of calligraphy, seals are very intriguing objects. Their imprints on the paper serve as the "signature" of the calligrapher and by this can we identify the artist who brushed the calligraphy. The characters on a seal can also represent the school or the monastery the calligrapher belongs to or it can express a thought he considers very important. While in the past calligraphers made their own seals, today there are expert seal-makers who specialize in creating these beautiful objects.
Finally let's examine four seals with the same name, Niko (仁虎) inscribed on them in four different styles (the last one in seal script).