A salt is an ionic compound that can be formed by the neutralization reaction of an acid and a base, to form the salt of the acid and water.
Metal oxides are bases, so they (generally) react with acids to form salts and water.
CaO+2HCl→CaCl2+H2O
In this reaction, the salt is CaCl22 (calcium chloride).
Metal sulphides, on the other hand, are salts formed by the reaction of metal bases (such as NaOH) with H2S, which acts as an acid.
H2S+2NaOH→Na2S+2H2O