1.essense is the set of properties which makes up an individual
'things' identity. its best thought of with relation to the 'essence-accident' distinction.
2.a substance is a basic, durable and fundamental 'thing' .
the different 'modes' (or forms) of a substance would be called properties of the substances 'kind' (physical substance has physical properties, mental substance has mental properties, etc)
- [note: there is debate on whether there is more than one substance,
and on what 'kinds' of substance there are]
3.existence is a hard term to pin down but in general, if there are a set of things which are 'logically possible' something exists if it is in the subset of those logical possibilities called 'actual' or at least 'real' things.
(the difference between the two comes from something called modal realism, which makes the conversation a little odd)
4.being and entity are often used interchangeably (along with words like 'object' and 'thing') a good way to think about them are as 'subjects of conception'. if you can conceive of it, its an entity. we would also say that if there is something which exists, but it can't be thought of, it is still an 'entity' or 'being'.
- [note: depending on the author, 'being' will more often be used to refer to living things and 'entity' to refer to animate things.]