Difference between "id" and "_id" fields in MongoDB

I expect it's just a typo in the documentation. The _id field is primary key for every document. It's called _id and is also accessible via id. Attempting to use an id key may result in a illegal ObjectId format error.

That section is just indicating that the automatically generated ObjectIDs start with a timestamp so it's possible to sort your documents automatically. This is pretty cool since the _id is automatically indexed in every collection. See http://www.mongodb.org/display/DOCS/Object+IDs for more information. Specifically under "BSON ObjectID Specification".

A BSON ObjectID is a 12-byte value consisting of a 4-byte timestamp (seconds since epoch), a 3-byte machine id, a 2-byte process id, and a 3-byte counter. Note that the timestamp and counter fields must be stored big endian unlike the rest of BSON.

The _id field is the default field for Bson ObjectId's and it is,by default, indexed.

_id and id are not the same. You may also choose to add a field called id if you want, but it will not be index unless you add an index.

It is just a typo in the docs.

id is an alias for _id in mongoid.id would return the _id of the document.https://github.com/mongodb/mongoid/blob/master/lib/mongoid/fields.rb#L47

if the _id field is not specified an ObjectedId is generated automatically.