Merhaba. 

Does "bu" stand for "this is" or just "this"? 

Teşekkürler.

what is the difference between "this is" and "this"?

Both.

For instance:

Bu tabak = this plate

Bu tabak = this is (a) plate

Which is used when? You are supposed to sense it.

Actually;

This = Bu

This is = Bu ....dır/tır.

But sometimes we do not use "..dır/tır" for daily life.

Examples:

A: hangisi güzel? (which one is nice)

B: Bu güzel (same meaning as "bu güzeldir")

If you use as adjective, "this". If you use as subject, "this is".

An ambiguity may happen in the short sentences:

Bu masa. (1This table. 2This is table.)

A: O ne? (What is it?)

B: O sandalye. (That is a chair.)

A: Bu? (This one?)

B: Bu masa. (This is a table.)

E: Salondaki en eski masa hangisi? (Which one is the oldest table in the saloon?)

F: Bu masa. (This table.)

_

Bu Cumartesi´den önce belli olmaz. (1It doesn´t become clear before this Saturday. 2This doesn´t become clear before Saturday.)

If it is the subject of the sentence, the ambiguity is solved by a comma.

Bu, Cumartesi´den önce belli olmaz. (This doesn´t become clear before Saturday.)

_

It is the same for other demonstrative pronouns too.

O odayı temizlemek için geldi. (1S/he came so as to clean the room. 2S/he came so as to clean that room.)

O, odayı temizlemek için geldi. (S/he came so as to clearn the room.)

Bu ders bittikten sonra kararlaştırılacak. (1This will be decided after the lesson finishes. 2One will decide about this after the lesson finishes.)

Bu, ders bittikten sonra kararlaştırılacak. (This will be decided after the lesson finishes.)

O sabah hiç konuşmadı. (1S/he never talked in that morning. 2S/he never talked in the morning.)

O, sabah hiç konuşmadı. (S/he never talked in the morning.)

=Sabah o hiç konuşmadı.

O sabah o hiç konuşmadı. (S/he never talked in that morning.)