Salat al-Duha and Salat al-Ishraq

Salat al-Ishraq - prayer begins ten to twenty minutes after sunrise
Salat al-Duha - prayer to be offered after sunrise and before midday

Abu Dharr reported Allah's messenger (may peace be upon him) as saying: In the morning charity is due from every bone in the body of every one of you. Every utterance of Allah's glorification is an act of charity. Every utterance of praise of Him is an act of charity, every utterance of profession of His Oneness is an act of charity, every utterance of profession of His Greatness is an act of charity, enjoining good is an act of charity, forbidding what is distreputable is an act of charity, and two rak'ahs which one prays in the forenoon will suffice.
Sahih Muslim, Book 004, Number 1557

Narrated Abu Huraira: My friend (the Prophet SAW) advised me to observe three things: to fast three days a month; to pray two Rakat of Duha prayer (fore-noon prayer); and to pray Witr before sleeping.
Bukhari, Volume 3, Book 31, Number 202 / Sahih Muslim, Book 004, Number 1558.
Another version can be found in
Bukhari, Volume 2, Book 21, Number 274 / Sahih Muslim, Book 004, Number 1561

And he who goes out to say the mid-morning (duha) prayer, and takes the trouble for this purpose, will take the reward like that of a person who performs umrah.
Abu-Dawood, Book 2, Number 558

For a more details, please view Shaykh Muhammad ibn Adam al-Kawthari's response here.

According to majority of scholars of Hadith and Fiqh, Duha Prayer, Al-Ishraq Prayer, and Al-Awwabin Prayer are all names of the same salah (ritual Prayer). A minority of scholars, however, have made a distinction between Al-Ishraq Prayer and Duha Prayer. According to them, Al-Ishraq Prayer is done after sun rise (around fifteen minutes after) while Duha Prayer is done much later.

Since it is an optional Prayer, there is no hard and fast rule to follow. It is enough to say in one’s mind that one is praying nafl or sunnah of Duha or Ishraq.