Istijmaar (Arabic - اسْتَجْمَرَ) means to cleanse the front or back passage of urine or stools using stones or something else in their place. One of the things that may take their place is tissue, but on the condition that one wipes no less than three times. And one should not use things that are forbidden to be used for istijmaar such as dung and bones, or things that have a kind of sanctity such as food etc.
Abu Hurairah(R) reported:The Messenger of Allahﷺsaid: When anyone performs ablution he must clean his nose and when anyone wipes himself with pebbles (after answering the call of nature) he must do that odd number of times. Sahih Al Bukhari Vol.1:161 Sahih Muslim 237, Sunan Nasai Vol. 1, Book 1, Hadith 88 and sunan Ibn Majah Vol. 1, Book 1, Hadith 409
When doing istijmaar using stones, it is stipulated that one should wipe three times or more in order to clean the place properly. Narrated by Salmaan al-Faarisi (may Allaah be pleased with him) said: “The Prophet (peace and blessings of Allaah be upon him) forbade us to clean ourselves using less than three stones.” Sahih Muslim 262
It is permissible to do istijmaar with or without water. The scholars said, it is better to use both, because that is cleaner. I’laam al-Musaafireen bi Ba’d Aadaab wa Ahkaam al-Safar wa maa yakhuss al-Mallaaheen al-Jawwiyyeen by Shaykh Muhammad ibn Saalih al-‘Uthaymeen, p. 10 
Doing istinja’ with water is better, because of the report narrated by Anas ibn Maalik (may Allaah be pleased with him) who said: When I was a boy, when the Messenger of Allaah (peace and blessings of Allaah be upon him) entered the toilet I would carry a small vessel of leather filled with water, and he would clean himself with water. Sahih Muslim 271 and Sunan Nasaa’i 45
Ibn Qudaamah (may Allaah have mercy on him) said in al-Mughni (1/206): If a person wants to do just one of them, then using water is better, because of the hadeeth that we have narrated, and because it purifies the place and removes the dirt itself and its traces, and is more cleansing.