Bhagavad Gita Verse 25, Chapter 4

daivamevaapare yajnyam yoginaha paryupaasate |
brahmaagnauvapare yajnyam yajnenaivopajuhyati || 25 ||
 
Some yogis also worship deities through devotion, other yogis engage in worship by offering the self into the sacrificial flame of the self.
 
daivam : deities
eva : also
apare : some
yajnyam : devotion
yoginaha : yogis
paryupaasate : worship
brahmaagnau : self in the form of sacrificial flame
apare : others
yajnyam : worship
yajnena : self
upajuhyati : engage
 
In the last shloka, Shri Krishna gave us the ultimate vision of a karma yogi. This person thinks of brahman in everything and every action that he performs; it is 24/7, every second. For seekers who have just begin their journey, such a grand vision is difficult to consistently maintain. So in the following shlokas, Shri Krishna gives us simpler forms of worship for practice, so that we can ultimately build ourselves up to this grand vision.
 
For most of us it is difficult to comprehend the formless brahman. Therefore, many yogis worship a deity, in other words they invoke Ishvaraa in a particular idol or photograph. This offering of services to Ishvaraa is called "pooja". Now in pooja, imagination plays an important role. Just like children play the house house game - the better the imagination, better they enjoy it. The process of pooja helps us develop rapport with Ishvaraa.
 
Once we get into the habit of pooja, we then imagine Ishvaraa in every action that we perform. We can even imagine Ishvaraa in the form of our friend, brother, parents and so on. It helps us to purify our mind and reduce fear caused by our attachments. As our trust in Ishvaraa increases, our fear decreases and we become calmer.
 
In addition to seekers who worship Ishvaraa, there is another group of seekers that are more thought oriented and more contemplative. They strive to realize the formless brahman through inquiry, contemplation, meditation, and by understanding their oneness with brahman. This form of worship is called jnyaana saadhanaa or spiritual practice through knowledge. It is like the wave surrendering completely to the ocean to realize its oneness with the ocean.
 
As we study the different forms of yagnyas that we can practice, it is easy to get carried away by the variety and the details. But the key point to remember is this. Any action we perform as part of a yagnya must not ever have even a tinge of expectation of result. If that happens, the yagnya becomes an ordinary, selfish action. The goal is to get better at these practice yagnyas so that we can bring that yagnya spirit into all actions in our daily life.