Shape determines function. This is true for everything. It is especially true for your muscles. There are over 650 muscles in the human body and each one has a unique shape and function.
In parallel muscles, the fibers are arranged in a parallel manner the length of the muscle. This arrangement allows for a greater range-of-motion than a similar sized pinnate muscle.
The subgroups of parallel muscle include flat muscles (the obliques), fusiform (biceps brachii), strap (the sartorius), and convergent (the pectoralis major).
Pennate muscles have fibers that attach to its tendons at an angle (opposed to the parallel fiber orientation). This formation allows for a greater production of force due to a greater cross-sectional area.
The subgroups of pinnate muscle are unipennate (the biceps femoris), bipennate (the rectus femoris) and multipennate (the deltoids).
(picture is from Plixs.com)