Difference between CRM Discovery Service and CRM Metadata service?

CRM Discovery Service:

A single Microsoft Dynamics CRM installation can host multiple organizations on multiple servers therefore it is important to specify which organizations need to be accessed. The Discovery Web service returns a list of the organization the specified user belongs to and the URL endpoint address for each organization. It will only give organization related information.

CRM Metadata service

It is specific to CRM records. Use to interact with entities and attributes like create, update or delete, retrieve or add value to pick list. It holds information about entities and attributes.

• An index in a book is Metadata, it helps you categorise/segment the information
• Metadata is the characteristics of the data e.g. data type, length, description
• It can show the relationships between data
• Metadata allows efficiently classify and a structured way to organise data for ease of use

Use the IOrganizationService web service to read and write data or metadata.

One of the common uses for Metadata is to retrieve the string value of an optionset.  OptionSets are stored in the CRM database as Int numbers and the string values are in the Metadata.