What is Purusha?
Purusha is Pure Being. It is the ultimate subject or observer. If you were to close your eyes, you would notice the sensations of your physical body, thoughts, and maybe even emotions.
It may eventually occur to you that your body, your thoughts and your emotions are happening on their own, regardless of your participation. "You" are simply the "being" which observes or experiences the thoughts, the body, or emotions. This being is also known as the Self or Purusha.
If you look for this observer, you will notice it has no form, it simply watches. Because it has no shape of its own, it takes on the shape of whatever it observes.