Similar to a  traditional “press and curl” using a Marcel iron or hot comb, a silk press is done with a flat iron.  Also, the hair tends to have a lot of body and movement, be lighter and silkier, and exudes a healthy shine. Unlike silk presses, traditional presses are usually weighed down with pressing oils and heavy grease.
A silk press is done on natural hair only. Relaxed hair is already straighten through a chemical process. By using this process excessive heat from a press will only lead to extreme damage.
A silk press is an upgraded version of a traditional press and curl.  As with a press and curl, the hot comb is warmed up then used to press your hair to  straight.  Most of the time grease, or some greasy like substance, is used to help get the hair straight.  This also leaves the hair extremely heavy, with no body or movement, and looking like it is stuck to the head.
A silk press is a modern day press and curl that gives natural hair. The look like it has been relaxed. It allows the natural hair to be very straight, shiny, silky, and soft.  It is for natural hair only because relaxed hair is already straight. So the need to press it is not necessary.  A silk press on relaxed hair is just a flat iron.  Unlike a traditional press and comb, a silk press does not use a hot comb.  Instead, a ceramic flat iron is used.  Also, absolutely no grease is used with a silk press, although a light oil serum can be used to add shine without weighing the hair press