They are called Demonstrative because they demonstrate which item is being mentioned.

The Demonstrative Pronouns and Adjectives - bu - this, şu - that, o- that yonder - use buffer letter -n- to become bun-, sun-, on- when adding any futher suffixes.

NOTE: The Demonstrative Pronouns and Adjectives - this, that, that yonder - use buffer letter -n- when adding further suffixes in all cases.

These are called Demonstrative Pronouns and Demonstrative Adjectives because they demonstrate or describe which item is being talked about.

Note: Turkish has two words for - that.
ŞU - signifies something near by OR something between the speakers.
ŞU also means - that which follows- eg. şu tavsiye - the following recommendation.

O - signifies items far away or which does not lie between the speakers.
O - is often used for descriptions of happenings in foreign countries and cities.

Note: When adding any suffixes including the plural suffix -lar buffer letter -n- is always used with the Demonstratives - thus forming -nlar as the Plural Suffix.

Care must be taken using - şu - and its extensions as it is also used in a derogatory sense (according to context) - see below.

Şu adama bakın! - Look at that bloke!
Şunu istemedim - I didn't want that! - (damn thing)
Şu kahrolasıca heref kim? - Who the hell is that fellow?

These extended suffixed forms show the use of the buffer -n- which is always used with the demonstrative.

Note that the suffix -ca - concerning .. - is used with the Demonstrative Pronoun to produce - bunca - all this, bunlarca - all these, onca - all that, onlarca - all those.
Note also that - şunca - or - şunlarca - are not normally in use

Note: that although - şunca - or - şunlarca - are not normally in use - however - it is sometimes used dispargingly. However, in Turkish it could be used to despise something in quantity or magnitude , or someone in muscular power.
For instance, if someone has a small wound due to some reason and complaining too much about it then you could say - şunca/şuncacik, yaradan ölmezsin - You won't die from such a small wound (surely not..!.)

Şu ne? - What is that (just there)?

The answer - Şu bir gazete - That is a newspaper - is incorrect.
The correct answer is - O bir gazete - as the answer to the question Şu ne?.

Because after you ask the question - Şu ne? - the person being asks now knows where and what the subject actually is, therefore the answer must be something like - O bir gazete.

But as a phrase in its own right you can say such as - Şu bir gazete - That (just there) is a newspaper - but not as an answer to a question like - Şu ne? What's that?

For example: There are some of your friends in your home and a newspaper (gazete) on the desk in this case,there are two posibilities:

(1) If you are asked - Şu masanin üstündeki nedir? - What is it on that table? then you must answer it like - O bir gazete or just - gazete - not - Şu bir gazete

(2) If you want them to know that there is a newspaper, then you must say - Şu bir gazete - and here if they want to ask you where it is they must ask it like - Nerede o gazete? - not - Nerede şu gazete?.

Summary
Question: Şu ne?
Answer: O bir gazete.

Şu bir gazete - Cannot be a question..
But if it causes a question like - Where is that newspaper? - then it must be:
- Nerede o gazete? - NOT - Nerede şu gazete?.

Onlarca kiz or Onca kiz. If you say: onlarca kiz - we understand there are some girls but they are in some groups and each group (has say) 10 girls.

There are further derivations of - bu, şu, o - which have produced the words - böyle, şöyle, öyle - the meanings are a follows:

böyle - In this way/thus
şöyle - like this/ that
öyle - such as that/like that

The -ce suffix can also be added to form adverbs:

böylece - In this way/thus
öylece - such as that/like that

Böyle bir iş yapmayın! - Don't do anything like that!
Böylece onu yapın! - Do it this way..

şöyle böyle - Like this, Like that - (as French - "comme ci comme ca")

When asked How are you? - then şöyle böyle can be given as an answer - "So so... I'm up and down... I'm getting on OK.. etc.."

öyle bir sağanak yağmur yağdı ki.. - There was such a downpour that..

The "Formula Speak" - Öyle mi? - is also used a lot when listening with apparent disbelief to some one describing events. It means something like - Well I never.., Go on then.., Really??... - This formula is used a lot in normal daily conversation.