Logo, Signet or Monogram from one hand
A Logo is a sign that represent your brand in different issues of daily business. That's why the recognition factor and the simplicity of a logo are important. It has to work on different materials and in different sizes and colors and black and white as well as during the design phase.
There are different types of logos. The word-image mark, figurative mark, word mark. Companies familiar to consumers generally no longer need the text in the logo, which can be addressed with sufficient degree of familiarity in the redesign. For new start-ups, a word-image mark or just a word mark is recommended.
On the other hand, a signet is strictly a visual sign made of usually simple shapes to increase the comprehensibility. In that case it is possible to say that it is a figurative mark.
Under a monogram, people originally understood the single letters of the personal name. Over time, the term was used to refer to artfully designed letters that were put together to form a single character.