Edited synopsis

Author: matulef

draft/core/indexes.txt

@@ -7,34 +7,37 @@ Index Overview
 Synopsis
 --------
 
-Indexes are an internal representation of the documents in your
-database organized so that MongoDB can use them to quickly locate
-documents and fulfill queries very efficiently. Fundamentally, indexes
+An index is a data structure that allows you to quickly locate documents 
+based on the values stored in certain specified fields. Fundamentally, indexes
 in MongoDB are similar to indexes in other database systems. MongoDB
 supports indexes on any field or sub-field contained in documents
 within a MongoDB collection. Consider the following core features of
 indexes:
 
 - MongoDB defines indexes on a per-:term:`collection` level.
 
-- Every query (including update operations,) can use one and only one
-  index. The query optimizer determines, empirically, the best query
-  plan and indexes to use on a specific query, but can be overridden
-  using the :func:`cursor.hint()` method. However, :ref:`compound
-  indexes <index-type-compound>` make it possible to include multiple
-  fields in a single index.
-
 - Indexes often dramatically increase the performance of queries;
   however, each index creates a slight overhead for every write
   operation.
 
+- Every query (including update operations) use one and only one
+  index. The query optimizer determines which index to use 
+  empirically, by occasionally running multiple query plans, 
+  and tracking the most performant index for each query type. 
+  The query optimizer's choice can be overridden
+  using the :func:`cursor.hint()` method. 
+
+- Indexes can be created over a single field, or multiple fields using a 
+  :ref:`compound index <index-type-compound>`.
+
 - Queries that are "covered" by the index return more quickly
-  than documents that have to scan many individual documents.
+  than queries that have to scan many individual documents. An index is 
+  considered to "cover" a query if all the data that the query must return
+  is stored in within the keys of the index.  
 
-- By using queries with good index coverage, it possible for MongoDB
-  to only store the index itself and the most often used documents in
-  memory, which can maximize database capacity, performance and
-  throughput.
+- Using queries with good index coverage will reduce the number of full 
+  documents that MongoDB needs to store in memory, thus maximizing database 
+  performance and throughput.
 
 Continue reading for a complete overview of indexes in MongoDB,
 including the :ref:`types of indexes <index-types>`, basic