These two adjectives have related meanings, but they are used somewhat differently.
Impractical may be the easier one to remember, and more common; it means “not practical” where “practical” means “relating to, or able to be put into, action or reality”:
Many people believe it is impractical to stop using oil as fuel, but we need to do it.
Impracticable means “difficult or impossible to implement, or put into action or reality”:
I would like to help everyone reach their dreams, but that is impracticable.
Although these meanings are the same in some cases, there can be a subtle and important difference.
Impractical describes something which is, essentially, by its nature, difficult to put into action, or not worth putting into action, while something may be impracticable for other reasons. For example, it may be impractical to walk from Los Angeles to New York, but it is not impracticable; you can actually do it. And it is impracticable to use an airplane on the moon, because there is no atmosphere, but it is not impractical to use an airplane, in general.