MongoDB – Difference between index on text field and text index?

The two index options are very different.

• When you create a regular index on a string field it indexes the
  entire value in the string. Mostly useful for single word strings
  (like a username for logins) where you can match exactly.

• A text index on the other hand will tokenize and stem the content of
  the field. So it will break the string into individual words or
  tokens, and will further reduce them to their stems so that variants
  of the same word will match (“talk” matching “talks”, “talked” and
  “talking” for example, as “talk” is a stem of all three). Mostly
  useful for true text (sentences, paragraphs, etc).

  Text Search

  Text search supports the search of string content in documents of a
  collection. MongoDB provides the $text operator to perform text search
  in queries and in aggregation pipelines.

  The text search process:

  tokenizes and stems the search term(s) during both the index creation and the text command execution.
  assigns a score to each document that contains the search term in the indexed fields. The score determines the relevance of a document to a given search query.
  

  The $text operator can search for words and phrases. The query matches
  on the complete stemmed words. For example, if a document field
  contains the word blueberry, a search on the term blue will not match
  the document. However, a search on either blueberry or blueberries
  will match.

• $regex searches can be used with regular indexes on string fields, to
  provide some pattern matching and wildcard search. Not a terribly
  effective user of indexes but it will use indexes where it can:

  If an index exists for the field, then MongoDB matches the regular
  expression against the values in the index, which can be faster than a
  collection scan. Further optimization can occur if the regular
  expression is a “prefix expression”, which means that all potential
  matches start with the same string. This allows MongoDB to construct a
  “range” from that prefix and only match against those values from the
  index that fall within that range.

http://docs.mongodb.org/manual/core/index-text/

http://docs.mongodb.org/manual/reference/operator/query/regex/