There are different Usages of the letter S.
1. the nineteenth letter of the English alphabet
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
This letter denotes the next after R in a set of items, categories, sizes, etc. e.g. It also denotes the nineteenth of nineteen or more hypothetical people or things.
s = distance as in Mathematical formulae. s = second as sixtieth of a minute in SI unit of time measurement. S = Svedberg which is also a unit of time where 1S = 10-13s and is used to express sedimentation coefficient. Besides, S letter of the English Alphabet is used in Sweden as the International Vehicle Registration.
In Chemistry S = Sulphur the 16th element of the periodic table and in Physics S = Siemens as SI unit of Conductance.
The most Common Noun which is used to learn this letter at the Nursery level is Ship; S for Ship.