Lord Rama was an avatar of Lord Vishnu in Treta Yuga. Similarly , Shri Krishna was an incarnation of Lord Vishnu in Dvapara Yuga . According to various religious texts and ancient books like Vishnupuran , the 10th Avatar of Lord Vishnu will be called Kalki avatar and it will appear in Kalyug.
He is described as fierce, riding on a white horse ,with a sword in his hand in Vaishnava texts. But in fact his appearance is not described as pleasant, because Kalki avatar will appear on Earth when sins are on their peak , when Earth is close to its end , when the humans will forget all relationships and would be selfish and greedy. Then he will come to end the world and retain peace.
Some people ask about when and where Lord Kalki will be born. Well ,we don't know whether he will come or not , but we can say that all of the above can't be ignored just because we have very less proof about it.