ER-11 is the collet for the quietcut spindle. In short, it is the bit holder specification. The ER-11 collet can hold a bunch of different shanks / bits, from 1/8" (which ships stock), 1/4", and 1-5mm. Assuming you’re in the US, most traditional router bits are 1/4", where Dremel bits are 1/8".
As for bits, everyone has their favorites - A good thread is here.