These are an interesting little feature for when you get to level 30 (will be lvl 20 when F2P comes to EU servers). Basically you get to defend an instanced area (e.g. Gondamon) from waves of attacks with the help of your soldier. A detailed guide can be found here and I found this list of class/soldier combos quite helpful.
I was a bit put off these at first because I thought they might be PvP as the ‘join skirmish’ functions seem very similar to EQ2’s PvP Battlegrounds – but they’re not 😉 I have only done about 3 solo skirmishes so far but they were great fun. I’ve come across some really fantastic looking mobs and time spent in challenging slaughter is always fun 😉
I chose a Warrior soldier to start with but after reading some advice it seems that a Sage or Herbalist will make the instances a bit easier for my Hunter so I think I will switch. I imagine that it can be very grindy and boring if you do these a lot but they are a nice change of pace from questing.