What is the difference between ruqya and sihr?

They both concern the unseen, and that is where the similarity ends. They are opposites in every way that matters.

Source

Ruqya draws on the speech of Allah. Sihr depends on pacts with devils, and those pacts are almost always sealed by acts of disbelief: desecrating the Quran, offering sacrifice to other than Allah, or deliberate filth.

Method

Ruqya is open. You can watch it, record it, learn it, repeat it. Sihr operates through secrecy, unknown symbols, knotted cords, buried objects and hidden names.

Outcome

Ruqya seeks to remove harm. Sihr exists to cause it, or to force an outcome that would not otherwise happen.

One is worship. The other is, by scholarly consensus, among the gravest sins in Islam.