What is the major difference between SAPUI5 and SAP Fiori?

 The difference is that Fiori is a design concept, and SAPUI5 is a library framework. You can still build custom UI5 Apps that do not adhere to the SAP Fiori design guielines, but they are called custom UI5 Apps. You can follow reference purposes this video:

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SAPUI5 is technology whereas Fiori is a methodology. SAPUI5 is Hybrid app (Because of HTML5).