A milonga is an Argentine Tango social dance. People come as individuals or with a partner. Music is usually played in tandas, or sets, with 3 or 4 songs in a tanda. In betwen tandas there will be a cortina, a short music interlude (30 sec or so long) of non-tango music. This is the customary time to change partners or take a break if you want to.
A practica is a practice dance. Floorcraft protocols are generally relaxed as compared to a milonga. Specifically, it is OK to talk during the dance, and impeding traffic within reason while you work on a step is accepted.