Hoverbover,
Put simply, icing conditions are conditions where icing will build up somewhere on your aircraft. Depending on aircraft type it may be the engine intakes or elsewhere on the airframe that is most critical. It isn't possible to generalise too much because each type is different. Icing trials have to be carried out to determine limits / clearances for each type of aircraft.
Ice accretion rate depends on:
Temperature: Above OATs of zero C there is unlikely to be airframe ice, but engine intake pressure drop may cause local temperature drop to zero or below; hence a requirement for engine anti-ice on around 5 degrees C for turbine aircraft. Local heating (e.g. of the outer part of main rotor blades by air friction) may delay ice accretion on some parts of the airframe. Be aware that well below zero c precipitation can remain unfrozen till it hits your aircraft (down to minus 10 rings a bell but I might be wrong - if so someone please correct me.
Total water content: Simple, more water = more ice.
Droplet size: Affects the rate and type of accretion. Small droplets tend to freeze rapidly and form whitish (trapped air), low density, brittle ice. Large, supercooled drops are perhaps most dangerous as they freeze relatively slowly on airframe contact (contain more latent heat) and flow backwards in the airflow. This type of clear ice is "sticky" i.e. adheres more strongly to the surface and it is relatively dense and heavy. Rain ice is an extreme form.
Airspeed: Increasing airspeed cause more rapid droplet contact and increased accretion rate until about 400 kts when surface heating by air friction takes over and reduces it again.
Shape of the affected surface: Droplets have inertia and they cannot change direction as rapidly as the air supporting them. Thinner objects tend to collect more ice. Wider, more rounded ones collect less in the same conditions as the droplets can smoothly follow a gradual path change, although ice may form very rapidly at a stagnation point at the front of an object e.g. on the aircraft nose or ahead of a fairing, depending on design. The windscreen wiper arms are often a good indicator of what might be happening further back on the airframe. Some aircraft have an ice accretion probe (with a thin profile) fitted where the pilot can see it. Flight in icing conditions may then be allowed by the aircraft manufacturer until a certain ice build up occurs.
To my knowledge, the Super Puma is still the only helicopter cleared for unlimited ops in icing conditions. It has electrically heated blades, windscreens, intakes and other surfaces powered by an extra generator, covers over the control rods to main and tail rotors etc etc.
The best advice for helicopters and icing conditions is to vacate the conditions or adopt the "LOALI" principle.
"Land On And Leg It"!