who were the equestrians?

[Emerodius]

So i want to know where exactly do they fall in the Patrician/Plebeian Spectrum? I know that originally they were pople who posessed a horse and served in the cavalry but what about towards the late republic/early empire era? I always thought that Equestrians were (at that point) were mostly people who were of plebeian ancestry but had managed to make money and thus were enrolled in the equestrian order. is this rifght or wrong? Were they allowed in the senate? were they considered a subset of plebs and thus were their interests represented by the Plebiean tribune?