Bhagavad Gita Verse 36, Chapter 18

sukham tvidaaneem trividham shrunu me bharatarshabha |
abhyaasaadramate yatra duhkhaantam cha nigachhati || 36 ||
 
Now, listen also to the three types of joy from me, O foremost among Bharatas, in which one enjoys its practice and attains the end of sorrow.
 
sukham : joy
tu : also
idaaneem : now
trividham : three types
shrunu : listen
me : from me
bharatarshabha : O foremost among Bharatas
abhyaasaat : practice
ramate : enjoys
yatra : in which
duhkhaantam : end of sorrow
cha : and
nigachhati : attains
 
At the end of the day, the end goal of any endeavour or action is to eliminate some type of sorrow, whether it is in the short term to remove hunger, or it is in the long term to prevent financial instability in our family. The lifecycle of an action begins with Vaasanaas. These Vaasanaas or deep rooted impressions create thoughts, some of these thoughts become desires, and consequently, desires become actions. When the action is complete and the target of the action is attained, the desire subsides, and the mind is free of desires for a split second. This stillness of the mind results in joy.
 
Shri Krishna says that even this joy obtained as the result of an action is in the realm of Prakrirti. Any by product of an action is in the realm of Prakriti since actions themselves are in Prakriti. So therefore, this joy can also be classified into three types, which are saattvic, raajasic and taamasic. This also mean that the type of joy obtained is closely related to the knowledge, doer and action behind obtaining that joy. A taamasic action will not result in saattvic joy.
 
Shri Krishna also adds that the complete end of sorrow is only obtained through saattvic joy. This is because the other two types of joy, raajasic and taamasic, are mixed and impure respectively. They either have a tinge of sattva, or none at all. Furthermore, saattvic joy is such that having tasted it even a little bit, one becomes so attracted to it that one delights in performing actions that result in sattvic joy. That is why, saattvic joy is described in detail in the next shloka.