The word schema is a generic term for a document that specifies the layout and permissible content of a class of documents. It actually entered computer science in the context of database schemas. For XML, there are multiple different schema languages with their own strengths and weaknesses, including DTDs, RELAX NG, Schematron, and, of course, the W3C XML Schema Language.
There is a tendency among developers to use only the word schemas, or perhaps the only slightly less generic XML Schemas, when referring to the W3C XML Schema Language. This needs to be resisted because the W3C XML Schema Language is neither the only such language nor, in most people's opinions , the simplest, most powerful, or best designed. It is merely one language promulgated by one group of inventors. It has some good points and some bad points, but we should not implicitly ignore all the other languages (some of which are demonstrably simpler and/or more powerful than the W3C XML Schema Language) by using the generic term to refer to the specific.
Unfortunately, the W3C has not chosen to assign its schema language an appellation less cumbersome than "W3C XML Schema Language." Consequently, to avoid repeating this phrase incessantly, I will occasionally succumb to temptation and use the word schemas to mean the W3C XML Schema Language. However, I will only do this in those items that discuss this language exclusively, and I will make it very clear at the outset of the chapter. Think of the word schema as more of a pronoun for the schema language currently being discussed than as a proper noun for the W3C's entry into the field.
Words have meanings. XML is a very precisely defined language, so its words have very precise meanings. It pays to use those words correctly. There are indeed some confusing aspects to XML. It doesn't make sense to make the problem worse by adding to the confusion. Using the right words for the right concepts can simplify many unnecessarily complex problems to save brain power for the things that are genuinely difficult.