The nature of trades unions under both capitalism and socialism is a question which has occupied many sections of the international Marxist movement since the beginning of the 20th century and before. The basics are, of course, obvious.
Trades unions are formed where workers unite to defend their immediate interests against those of capital. In fact, the very existence of trades unions is due to the basic contradiction between labour and capital. The primary motivation of capital is to increase the surplus value extracted from the workers.