Yōga Sūtra 2.01

तपःस्वाध्यायेश्वरप्रणिधानानि क्रियायोगः।

tapaḥsvādhyāyēśvarapraṇidhānāni kriyāyōgaḥ।

[तपः स्वाध्याय इश्वर प्रणिधानानि क्रिया योगः।
tapaḥ = austerity or exertion to internalize gained knowledge; svādhyāya = self-study; iśvara = īswara; praṇidhānāni = submission; kriyā = action; yōgaḥ = Yōga;]

The path of Yōga with action (kriya yōga) consists of internalizing the gained knowledge with practice (tapa), self-study (svādhya), and devotion to īswara.

Real kriya yoga is described here by Pātānjali. There are several others who have made the kriya yoga term famous, though they teach their own methods contrary to what is prescribed by Pātānjali. It's three foundation pillars are practice, self-study and submission to īswara. The attributes of īswara are already provided in first section.