Ijtema means 'gathering' and Johr is a smaller Ijtema.
During the time of the Prophet Muhammad peace be upon him, Muslims used to gather regularly in the house of Arqam behind Mount Safa. These days there is an escalator called 'Arqam Escalator' to show where the house of Arqam was. This was the place where Umar Ibn Khattab accepted Islam.
It has been reported that Hadhrat Arqam's house was at mount Safa and it was here that RasulAllah used to be found during the early days of Islam. It was here that he invited people towards Islam, as a result of which many people accepted Islam.
( Mustadrak vol. 3 pg. 502)
Hadhrat Ali reports that on 3 consecutive days RasulAllah sallallahu alaihi waSallam instructed him to prepare a goat with a saa (approximately 3.6 kg) of grain and a large container of milk to host the family of Abdul Muttalib, so that RasulAllah could invite them to Islam.
( Ahmad as quoted in Tafseer Ibn Katheer vol.3 pg350)
This gathering in the house of Arqam was one of Muslims alone, where learning and teaching of Islam took place. RasulAllah sallallahu alaihi waSallam also had food served to the people he gave Da'wah to. All the above takes place in an Ijtema or johr. In some countries, to facilitate huge crowds, emergency toilets are erected, tents are constructed and small food shops are also set up. In other countries, people even prepare there own food. When the Usul (principle) is found, all that happens for organizational purposes, as long as within the boundaries of Islam, is permissible.
(Mufti Afzal Hoosen Elias)