Although digital cameras can be set to compensate for various color temperatures internally (basically by adjusting the weighting of RGB channels prior to saving an image) they can still benefit from filtration. Filtration balances or alters light BEFORE it hits the sensor, whereas adjusting color balance in the camera amplifies or partially ignores certain color channels AFTER it has been recorded. For small changes, in-camera adjustment is adequate. For large changes, filtration may be superior.
The "effective focal length" is best described as a "field of view crop". Most current digicams have a sensor smaller than a 35 mm film frame so you are viewing a portion of the image that would otherwise fully cover a 35 mm film frame. Digital SLRs crop almost half the image, which is effectively the same as the field of view of a lens 1.5X the focal length on a 35 mm film camera. It's a crop out of the middle of a 35 mm frame.