Missiles with the [SEAD] tag can target radar AA that has been left on, and nothing else (well, ships too, but the damage is lacking). With the exception of the ka-52 recon helictoper, they’re exclusive to planes.
How to use them? First, don’t be too aggressive. Yes, this applies to all planes, even the ones that counter AA. While SEAD comes with the highest ECM values you can get for the price, sending them directly at the enemy AA will eventually force them to fly over and past it. SEAD can only shoot what is directly ahead, so you can expect all radar AA to turn on and shoot back as soon as the plane flies over or tries to evac. Getting too close also exposes the plane to deadly short-range IR AA.
Instead, know where the enemy AA is likely to be and start turning away as soon as you start getting in range. This allows the plane to shoot at any radar that is on while minimizing the time it spends exposed to return fire.
Second, don’t think of SEAD as a way to earn destruction points. Better players will keep radar AA off until they need it, making SEAD-only runs pure risk, with no reward. SEAD’s job is to protect another plane, sending it alone is usually pointless. The correct usage is to send your bomber to do its task as usual, and time a SEAD plane to arrive on the scene just after that. Having the SEAD arrive first means it will start evacuating while the more fragile bomber is still on the field, exposing it to AA. Having the SEAD arrive later will allow it to cover the bomber’s evac, and potentially even bait the enemy into turning their AA on.
How do you deal with SEAD? The direct counter is to intercept the enemy bomber train with a fighter, rendering the SEAD meaningless. The indirect one is to find a way to position IR AA close enough to get kills. Planes fly at different altitudes, with SEAD and fighters usually being very high up, so getting IR in range is extra hard against SEAD. Finally, bigger numbers of heavy SAMs can reliably down SEAD as it turns away after strikes.
If you want more on the topic, here’s Razzman showing how to juke SEAD missiles by toggling radar AA (juking is after 4:00, but he also shows some SEAD usage before that):