Ported slides means pretty much exactly as it sounds. Ports are machined in to the slide, essentially, to lighten the weight for faster cycling.
Generally, you will only find this on race / target / match pistols, because as mentioned, there is greater chance for dirt entry.
Ported barrels, for real world use, are generally aimed upwards, near the muzzle of the barrel. The purpose is twofold:
- Blows off expanding gas behind the projectile so that as it leaves the muzzle, it will not be disrupted by gas expanding faster than the moving projectile. Essentially, it makes that individual shot more accurate. This does not apply to all ported barrel designs. Only some are designed for this.
- Compensates for muzzle flip by blowing gas (energy) upwards. Essentially, it allows for faster follow up shots.
In airsoft, a ported outer barrel does nothing except look bad ass:
The exception to that, is if the inner barrel is also ported, which aids in stabilizing the round as mentioned above, by reducing back pressure before the round leaves the muzzle. However, the inner barrel ports must also mate up with the outer barrel ports for this to be effective.