What is the difference between combining and shortening prayers, and when is each of them legislated? Is it permissible to combine and shorten the 'Isha prayer with the Maghrib prayer while traveling? And what is best for the traveler regarding the 'Isha prayer?

Jamʿ (combining) is performing the Dhuhr and Asr prayers together during the time of one of them, and similarly for Maghrib and Isha. This is a permissible concession for the traveler. As for Qasr (shortening), it is reducing the four-rak'ah prayer to two rak'ahs during travel. This is a Sunnah that is better than performing the full prayer, due to the Prophet (peace and blessings of Allah be upon him) consistently doing it. Combining and shortening prayers are prescribed when one departs from the built-up area of a town by a distance of no less than 83 kilometers. Combining and shortening prayers are not obligatory, and shortening is preferable.