Shaf’ in Arabic refers to pairs or even numbers, and is the opposite of witr which means odd-numbered. The proven Sunnah prayers that are to be offered after ‘Isha’ prayer are of three types: 

1-The Sunnah prayer after ‘Isha’ – two rak’ahs

2-Qiyaam al-layl, during which one may pray however many rak’ahs one wishes, two by two.

3-Witr – which may be offered with one rak'ah or with three, five, seven or nine. 

See the answer to question no. 46544 

If a person chooses to pray Witr with three rak'ahs, he may either pray them one after another with one tashahhud, or he may pray two rak’ahs then say the salaam, then pray one rak’ah. 

The majority of scholars are of the view that the least proper form of Witr is to pray three rak’ahs, whether one does that joined together or separately. Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) said: “The least perfect of Witr is to pray two rak’ahs and say the salaam, then perform one rak'ah and say the salaam. It is permissible to do it with one salaam, but with one tashahhud and not two, because if you do it with two tashahhuds it will resemble Maghrib prayer, and the Prophet (peace and blessings of Allaah be upon him) forbade making it resemble Maghrib prayer. End quote from al-Sharh al-Mumti’, 4/21 

Ibn Hibbaan (2435) narrated from Ibn ‘Umar (may Allaah be pleased with them) that he used to separate between his even-numbered prayer (shaf’) and odd-numbered prayer (witr) with a tasleem, and he said that the Prophet (peace and blessings of Allaah be upon him) used to do that. Al-Haafiz said in al-Fath (2/482): its isnaad is qawiy (strong) 

This hadeeth indicates that what is meant by al-shaf’ is two rak'ahs before the one rak'ah of witr.

Shaykh al-Albaani (may Allaah have mercy on him) said in his essay on Taraweeh prayer, after describing the various characteristics of Witr prayer that are narrated in the Sunnah: 

To sum up: from the above we see that praying Witr in any of these ways is permissible. Praying three rak'ahs with two tashahhuds like Maghrib prayer is not mentioned in any saheeh hadeeth, rather it is makrooh. Hence our view is that one should not sit between the two rak’ahs and the one rak'ah; if a person sits he should say the salaam. This is better. End quote. 

Some people think that al-shaf’ refers to the Sunnah prayer that follows ‘Isha’, but that is not correct.

It says in Fataawa al-Lajnah al-Daa’imah (7/255): 

The Sunnah prayer that comes after ‘Isha’ is two rak’ahs, unlike al-shaf’ and al-witr. End quote. 

And Allaah knows best.