There are many differences between joining and shortening prayers, including the following:
The meaning of shortening prayers is that the four-rak’ah prayers become two rak’ahs when travelling.
As for joining prayers, it means that the worshipper joins two prayers, Zuhr and ‘Asr, or Maghrib and ‘Isha’, at the time of the earlier or later of the two prayers.
2. Shar’i ruling
The scholars are unanimously agreed that shortening the prayers is better for the traveller than offering them in full, because the Prophet (peace and blessings of Allaah be upon him) shortened prayers during all his journeys, and there is no saheeh report that he offered the prayers in full whilst travelling.
Ibn ‘Umar (may Allaah be pleased with him) said: I accompanied the Messenger of Allaah (peace and blessings of Allaah be upon him) and he did not do more than two rak’ahs whilst travelling, and the same applies to Abu Bakr, ‘Umar and ‘Uthmaan (may Allaah be pleased with them). Narrated by al-Bukhaari (1102).
The Hanafis are of the view that it is obligatory for the traveller to shorten his prayers, but the correct view is that of the majority, that shortening the prayers is sunnah mu’akkadah (a confirmed Sunnah), and that it is better than offering the prayers in full.
See: al-Ijmaa’ by Ibn al-Mundhir (27); al-Mughni (1/382); al-Mawsoo’ah al-Fiqhiyyah (27/274).
As for joining prayers, the scholars are not agreed on its permissibility except in the case of the pilgrim in ‘Arafah and Muzdalifah. Some scholars said that it is not permissible to join prayers anywhere except in these two places.
The correct view is that of the majority of scholars, which is that it is permissible to join prayers if there is an excuse for doing so, because it is proven that the Prophet (peace and blessings of Allaah be upon him) did that in places other than ‘Arafah and Muzdalifah.
3. Reasons which make it permissible to join or shorten prayers
The reasons which make it permissible to join prayers are broader than those which make it permissible to shorten them. Joining prayers is permissible for every traveller, and for the non-traveller if it is too difficult for him to offer every prayer on time, such as one who is sick, or if there is rain, or he is busy with some work that he cannot delay in order to pray, such as a student taking an exam or a doctor who is doing surgery and so on.
With regard to shortening prayers, that is only permissible when travelling.
Shaykh al-Islam Ibn Taymiyah (may Allaah have mercy on him) said in Majmoo’ al-Fataawa (22/293):
The reason for shortening prayers is travelling only, and it is not permissible in situations other than travelling. As for joining prayers, the reason for it is need and excuses, so if a person needs to he may join both shortened and full-length prayers whilst travelling, and he may join prayers when it is raining and so on, or because of sickness and the like, and for other reasons, because the purpose behind it is to spare the ummah hardship. End quote.
Shaykh Ibn ‘Uthaymeen said in al-Liqa’ al-Shahri (60/11):
Joining prayers is broader in scope than shortening them, i.e., the reasons for doing so are more numerous.
And Allaah knows best.