Avatar or Avatara (अवतार, Avatāra), is incarnation of God on earth. It literally means descent (avatarati) and usually implies a deliberate descent from higher spiritual realms to lower realms of existence for special purposes. Lord Sri Krishna was one of those Avatars of Lord Vishnu. In the past there had been so many Avatars of Lord Vishnu. In Lord Sri Krishna's case you can call these as His previous births but with a special purpose to establish Dharma (righteousness) on this earth. Below are verses from Bhagwat Gita which explain that only:
"yada yada hi dharmasya
glanir bhavati bharata
tadatmanam srjamy aham" (Bhagwat Gita: Chapter Four verse 7)
vinasaya cha duskritam
sambhavami yuge yuge" (Bhagwat Gita: Chapter Four verse 8)
"Sri Krishna said to Arjuna: Whenever Dharma, or the situation of law and order, is endangered on this world, I incarnate onto this world to re establish Dharma, law and order, and to protect the Sadhus or saints and to destroy the evil elements of the society. I do so appear, millennium after millennium."
So in this way Lord Vishnu. descended on earth in Treta Yuga as Lord Rama & later on in Dwapar Yuga, Lord Vishnu descended on earth as Lord Krishna. In "Treta Yuga" Lord Vishnu appeared as Ramachandra, the prince and king of Ayodhya who killed demon king Ravana. The great epic Ramayana speaks of the glory of the Rama Avatar of Lord Vishnu. However the same Lord Vishnu appeared in "Dwapara Yuga as Lord Krishna along with his brother Balarama" Mahabharata speaks of Balarama and Krishna in detail.
In the "Bhagwat Gita Chapter Ten-Verse 31", "Lord Krishna" himself clearly says that "Among the wielders of weapons, He himself is Lord Rama."
"pavanah pavatam asmi
ramah shastra-bhrtam aham
jhasanam makaras chasmi
srotasam asmi jahnavi" (Bhagwat Gita: Chapter Ten verse 31)
"Sri Krishna says to Arjuna: Of purifiers I am the wind, of the wielders of weapons I am Rama, of fishes I am the shark, and of flowing rivers I am the Ganges."
So this verse clearly establishes that "Lord Rama" & "Lord Krishna" are the forms of Supreme God, "Lord Vishnu" only & hence are one & the same one being. So let's worship "Lord Rama" & "Lord Krishna" equally as both are incarnations of "Lord Vishnu" who came to this earth in different point of times in history to help the humanity.