(@hmouhtar moving this to a new thread since it’s an OpenMRS data model question.)
You can start reading here for example: Visits, Encounters and FHIR
… but there’s many many more threads, discussions, wiki pages and self-documenting code snippets that give hints as to what visits and encounters are in OpenMRS.
@burke I’m sure you could point to the ex-cathedrae explanation that you have given somewhere already? 
My layman take: a visit is made of a succession of clinical encounters occurring between its start date and its end date. Clinical data is recorded during those encounters: observations, conditions, diagnoses, allergies, drug orders, lab test orders, etc etc,. They are all attached to the encounter during which they were recorded.