What happens when we create index What happens exactly when I create an index on the empid column of this employee table

CREATE TABLE employee
(
  empid number(5),
  ename varchar2(30),
  sal   number(10,2)
);


Let's assume the table contains the below data 

empid   ename      sal
1       hema       10000
3       suma       10000
2       bheema     15000
4       bhama      20000
5       panama     25000


what exactly my doubt is what will be created and stored internally in index..... to improve the performance , whether sorted content of empid or some other information related to rows....?
Indexes, their various types and internal structures, are described in the documentation.