As the title. I've seen many definitions such as concealment refer to one or both, but I can't find a clear definition of them or how they are different.
Here's the definition of Line of Effect:
is a straight, unblocked path that indicates what a spell can affect. A line of effect is canceled by a solid barrier. It's like line of sight for ranged weapons, except that it's not blocked by fog, darkness, and other factors that limit normal sight. A line of effect starts from any corner of your square and extends to the limit of its range or until it strikes a barrier that would block it. A line-shaped spell affects all creatures in squares through which the line passes.
And here's the definition of Line of Sight:
is the same as a Line of Effect but with the additional restriction that that it is blocked by fog, darkness, and other factors that limit normal sight (such as Concealment).
This means that when a spell has a Target entry as part of its description
You must be able to see or touch the target, and you must specifically choose that target.
And this makes spells with a Target entry require line of sight (or being able to touch the target if appropriate). But if the spell creates an effect
You must designate the location where these [effects] are to appear, either by seeing it or defining it. Range determines how far away an effect can appear, but if the effect is mobile, after it appears it can move regardless of the spell's range.
Thus making those spells require only line of effect.
For example, a fireball spell can be cast into an area of darkness into which the caster cannot see but the caster must select a point of origin for the fireball's effect that's within the spell's range and to which the caster has line of effect. On the other hand, a charm person spell must be cast on a creature to which the caster has line of sight and line of effect as that spell targets a creature specifically.
More details can be found under Aiming a Spell.