There is a technical difference. In the USA (and elsewhere probably), some people are paid "salary" (per week, per month) and some people are paid an hourly wage. It would not be proper to use the term "salary increase"" to describe an increase in wages but "pay raise" could be used to describe both an increase in salary or an increase in the hourly wage.

There is no difference. We use both in English. However, "salary" increase is more "formal". The only difference could be between salary adjustment, salary increase, and a bonus. An adjustment is to balance one's salary with industry average, increase=raise, and bonus = 1 time "bonus". Let me know if you need more clarification.