Services module: What is 'Index' operation?
'Index' in services module will list out an entity(node, users, taxonomy..)  in descending Order of its created time, vid, name etc based on the arguments that are passed. The common arguments for the Index are 


page  - Page number of results to return (in pages of 20).
fields - The fields you want returned. 
parameters - An array containing fields and values used to build a sql WHERE clause indicating items to retrieve. (array)
pagesize - number of items to be returned. (Integer)

In case of retrieve, it returns the complete entity object of the passed 'entity_id' 


UPDATED:

Sample URLS for Index is 


http://domain/endpoint/node?fields=nid,vid&amp;parameters[nid]=7&amp;parameters[uid]=1&amp;page=2
http://domain/endpoint/taxonomy_term?fields=tid,name&amp;parameters[tid]=7&amp;parameters[vid]=1
http://domain/endpoint/user?fields=uid,name,mail&amp;parameters[uid]=1
http://domain/endpoint/node?pagesize=3


For documentation you can see Services: An API for remote applications.