A "frog" is the part of a switch or crossing where rails from separate tracks meet. In model railroading, frogs can be of two types: "insulated" (a.k.a. "dead", "plastic", or "insulfrogs") and "live" (a.k.a. "metal", or "electrofrogs").
Different brands/lines of N scale switches either have live or dead frogs, or in the case of Peco code 80, both. The difference is important enough that I'll refer to it a few times in future posts, so I'm covering it as a separate post here.
Here is a diagram of a switch with an insulated frog (yes, I made this myself):
The parts in the center colored in black are plastic -- or any other non-conductive material. The rails on the top (blue) are one polarity, no matter which way the switch is set, and the rails on the bottom (red) are the other. In order to keep the rails powered correctly the switch will have hidden wires, usually underneath the switch ties, connecting the rails as shown in the dotted lines.
By comparison, here is a diagram of a switch with a live frog:
All the rails, including the frog, are metal. Even though there is an apparent electrical gap between the rails at the frog itself, they are electrically connected across that gap.
Electrical power distribution is a bit non-obvious for live frog switches. For the moment, ignore the box on the right of the diagram labeled "SPDT" and the lines connected to the box. When you open a live frog switch out of the box the SPDT box won't be there. Instead, in the out-of-the-box situation the frog rails and all the inner rails will get their power from where the rail point touches the outside rail (in this diagram look immediately above the word "Rails" in red). Thus, when the switch is thrown the frog and inner rails will switch polarity from red to blue or vice versa.
The potential problem is that the electrical connection between the point and side rails can be tenuous -- and can get worse over time with dirt or corrosion. Many modelers choose to supplement the power to the frog and inner rails using an SPDT switch, as shown in the diagram -- and often this SPDT is built into a switch motor such as the Tortoise or even with a manual switch controller like the Blue Point.
The other issue is that the live frog switch always has both rails in the non-selected direction at the same polarity. Now, this can be useful in the DC world as a means of controlling multiple engines on the layout. A spur or single-ended siding that is attached to this switch gets no power unless the switch is set pointing in that direction. This function is called "power routing" and thus an alternate name for live frog switches is "power routing switches". (Note that the corresponding alternate name for dead frog switches is "all live" switches -- meaning every direction has both power polarities -- but the use of the term "all live" for a "dead frog" switch is so confusing that I avoid it.)
For most DCC layouts, which already have multiple engine capability, power routing is useless. DCC operators prefer the ability to move their engine on the spur or siding for tasks like car positioning -- even if the switch is set against them. Thus, DCC modelers will tend to run separate wire feeders to all track that is attached to the live frog switches, then add insulated gaps next to the frog rails to prevent shorts.
Except for power routing are there any other advantages to live frog switches? Well, there are two. First, they can be more operationally reliable. This is because the insulated frog creates a power gap when one of the wheels of an axle is in contact with it. With most modern locomotives that get power via all wheels this is not a problem, but with older locomotives or sometimes with short switchers with dirty wheels this can cause engine stalls. This can be easily solved by keeping wheels clean.
The second advantage is that the all-metal frog looks more realistic than plastic frog switches, even after they are painted and weathered. In a well maintained DCC layout this is your only real advantage of live frog switches.
So, yes, if you've followed this far you've realized that live frog switches have one advantage -- appearance -- and yet each live frog switch requires the addition of an SPDT switch, one extra feeder, and two insulated gaps. So is the hassle of all that extra work worth the marginally better appearance?
Unfortunately, many of us don't get the choice. If you choose code 55 rail all the switches have live frogs. Which is why my main layout is full of live frog switches.