An Initial Mark is one where the primary focus of the logo is on one or more stylized initials that represent the name of the organization. Most times an Initial Mark is a single letter which is the first letter of the company name. Digisaw below is a good example of this application.
However, just because the first letter Initial Mark is most common, having multiple letters for multiple words is not uncommon. Below, the company "MAC" stands for "Military Assistance Company". So MAC is the initials for all three words in the name.
While the initials are the primary focus of the logo, that is not to say that other graphic elements cannot play an important role in the overall design. SimPac is a good example of this. The "S", the initial, helps form the shape of the box and other elements complete the illusion.