Ishvara (Saguna or Personal aspect of God)
Brahman is the ocean of pure consciousness. When this ocean decides to create the universe It differentiates macrocosmically into three aspects:

Brahman
Transcendental God beyond creation - He is Nirguna that means the gunas are in an inactive state, Impersonal, Nameless and Formless.

Ishvara
Immanent God within creation - He is Saguna that means the gunas are in an active state. This aspect of Brahman that acts from within the universe is the personal God that most of the religions refer to. He is responsible for governing the process of creation, preservation and destruction of the universe. He still remains nameless, formless and is immanent in the universe.
However for the purpose of extending love, devotion and worship a devotee conceives Him in a particular name and form. In that name and form He is then understood to reside in the highest plane of existence within the universe. He doesn't have a particular name and form called God form. He is experienced, seen or heard in any form in which the devotee seeks Him. So God in a personal form can be worshipped in infinite ways not just one as some religions claim. Ishvara is the soul of the universe. The universe is the body of Ishvara.

Maya
God's Power or Energy that is used to create the universe. This universe of matter and energy is God in His lower or limited aspect. Maya is the Self limiting power of God. With this power God divides His ocean of cosmic consciousness into infinite individualised units of consciousness. Each such unit is called a Self or Soul. These individualized souls are like the waves in a ocean. God the one becomes God the many. God the ocean which is one and undivided becomes the waves which are many and divided. With the help of Maya God creates the universe bound by time, space and causation and limited by name and form and composed of mind, energy and matter. 

Maya has two aspects
Avarana Shakti
The power of veiling. With this power God creates the souls and makes them forget their divine nature.

Vikshepa Shakti
The energy of projecting or creating. With this energy God creates the universe and then places the souls within the universe. Within the universe the souls are given a body and mind with which they experience and evolve in the universe.

Concept of Atman and Jivatman (the Self and the Soul)
The Atman or the Self is a small part from the ocean of God's pure consciousness confined within the body. It is like the air confined in a bottle. The body (physical, astral and causal) is composed of mind, energy and matter. The body changes but the Self remains unchanged.