To determine Isha and Fajr calculations, one needs to calculate the time when the sun is at a certain distance below the horizon. The twilight angle is used for this purpose and it is defined as the angle in degrees of the sun below the horizon, when total darkness begins or ends.
Some organisations use a fixed time after Maghrib to determine the start of Isha time

The following standards exist for determining Fajr and Isha times:

Organisation	Fajr – twilight angle	Isha – twilight angle	Region
Muslim world league	18	17	Europe, Far East, parts of the USA
Egyptian General Authority of Survey	19.5	17.5	Africa, Syria, Iraq, Lebanon, Malaysia, parts of the USA
Islamic Society of North America (ISNA)	15	15	Parts of the USA, Canada, parts of the UK
University of Islamic sciences, Karachi	18	18	Pakistan, Bangladesh, India, Afghanistan, parts of Europe
Umm Al Qura	19	90 minutes after Maghrib (120 minutes during Ramadan)	The Arabian Peninsula

The above information was taken from http://www.ummah.net/astronomy/saltime/

The 1/7 rule is one of several rules for determining Fajr and Isha times. This rule divides the total time between sunset and sunrise into seven parts. Isha starts when the first seventh part ends (or when the 2nd seventh part begins) and Fajr starts when the last (seventh) part starts.
If Salahtimes determines that during the summer months, if total darkness does not occur, then the 1/7 rule is applied only if this results in later Fajr times, and/or earlier Isha times. This results is a smooth transition from the twilight angle used to the 1/7 rule. For prayer methods that specify a fixed time after Maghrib for Isha (or a fixed time before sunrise for Fajr) then the under these circumstances the 1/7 is not applied all.