YA:

a) it is not a word, it is a kind of exclamation which reflects your emotional or reactinoal mood. The sense of usage can be understood definitely only with the style of saying and in the context. And it's sense also varies according to your accentuation or saying expeditiously (like a quick "ya") or lenghty (like "yaaaaa"), though you can sometimes see both reactional and emotional mood in both short and long way of pronouncing.

Explanations can never be ebough, it can be comprehended only by practising and listening to those who use it.

 

Just some samples:   

 

- Ya sen deli misin?

(the person who says this has generally a bit feverish mood)

 

- George should come here to solve the problem.

- Ya gelmezse?

 

- Dnn akşam Tarkan'ı caddede g?rdnm.

- Yaaaa! Ne yapıyordu?

 

- Yaaaa! tok nznldnm.

(to strengthen the sadness)

 

b) as a definitive usage, it is "either ... or (ya ... ya da)"

- Bu yaz tatil itin ya Mars'a ya da Satnrn'e gitmek istiyorum (some imagination) 

 

a) it is rural and rude form of "evet" (you can more or less understand if a person is educated enough; or from city or village, taking his usage of "yes" as "evet" or "he" into account)

b) or just an exclamation like "ha!" (for example when you surprise)

 

var ya: word by word "it exists or there is!". But not quite like that in its usage, since "ya" gives it a slight difference. You use it generally at the begining of the sentence especially when you intend to do something decisively as a fact or found out about a fact from your estimations. When you say it, you generally consider what you are saying is a fact and "it exists". and only in spoken language.

 

- Var ya,  aslında bu adam yalan s?ylnyor.

 

 

- hani şu Tnrkte ?devi var ya, bence onu yapmayalım.

 

I may be considered a kind of teacher in fact