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In very simple words, XML is code written by you, and defined by you. In order to validate and process that code, a schema is written to support it.
I assume that a schema is a schema is a schema...which probably means that a database schema describes and constrains the content of a database...never heard of it before, but seems to make sense.
XML and Data definition Language or DDL(SQL Create,Add,Alter etc) are two common languages for documenting schemas, but can also use an entity relationship diagram or a data dictionary - typically a very large and hard to maintain spreadsheet :)
DDL has one huge benefit over the other methods - when you execute it, it creates the structure for you. But its not very good for communicating the schema to other (non-technical) stakeholders.