If we put it in a simple way, can we say generative grammar is about tree diagram, and transformation grammar is how sentences can be interpreted in another way?
The "generative" in "generative grammar" is defined by Chomsky in Aspects of the Theory of Syntax as meaning "explicit". Chomsky compares this sense of "generate" to its use in analytic geometry, when a mathematical function generates a curve. The calculation of the points on the curve for values of the variables in the function is explicit and well-defined.
There is no special relationship of "generative" to "transformational". Transformations in transformational generative grammar are simply one technical device used in TGG to calculate strings. Adding transformations to the theory is something like moving in analytic geometry from functions using only addition to those using also multiplication.
In the years since Aspects, variants of Chomsky's early formulation have emerged. An example of a modern theory that is transformational but not generative is McCawley's syntactic theory in The Syntactic Phenomena of English. An example of a theory that is generative but not transformational is GPSG -- Generalized Phrase Structure Grammar by Gazdar et al.
"Generative grammar" refers to a broad theory of language advocated by Chomsky and followers since the mid-60's. It includes research in syntax, semantics, phonology and morphology, and is still widely practiced. It's difficult to say exactly which theories are "generative" according to Chomsky's definition, since he is unclear about what he means, but the two essential and most resilient characteristics identifying such theories are (1) generative theories are mentalist (they make claims about the mind) and (2) they are (supposed to be) explicit and in principle formalizable. An alternative notion of "generative grammar", applicable only to syntax, is that GG is "whatever basic syntactic framework Chomsky currently espouses". But this is plainly inapplicable to other components of grammar.
"Transformational grammar" is a specific deceased theory of syntax, which used transformational rules – which were abandoned in the mid-70's. It is best represented in Aspects of the theory of syntax, and a compendium of such transformations and transformational analyses can be seen in From Deep to Surface Structure: An Introduction to Transformational Syntax by Marina K. Burt.