The practical function of the collar is access. A hospital ward at three in the morning, a police cordon, a prison, the family room after a road accident: in each of these a person in a collar is admitted with a speed that no explanation would achieve, because the garment does the explaining. Clergy describe this as the reason they keep wearing it in an age when almost nothing else about clerical dress survives outside a church.
The same visibility makes it a political instrument, and it has been used deliberately as one. Photographs of the American civil rights marches show ranks of collars at the front, worn by clergy who had travelled specifically so that the images would show them, and the same calculation is visible at protests since. A collar in a crowd changes how the crowd is read.
Two forms are in general use. The Roman style shows a small square tab of white at the front of a black clerical shirt. The Anglican or full band style shows a continuous white band around the whole neck, and was once a separate stiff collar fastened with studs, which is where the disparaging term dog collar comes from. Which is worn is now mostly a matter of taste rather than of church, though the full band remains more common in Britain.
Who may wear it varies. In the Roman Catholic Church it is for the ordained. Several Anglican provinces license lay readers to wear it, and practice in Protestant churches ranges from universal to unknown, with many Baptist and Pentecostal ministers never wearing one at all.
Impersonation is a real problem rather than a theoretical one, and some jurisdictions treat falsely presenting as a minister as an offence in itself where it is used to obtain access or money.