There are different ways of being eligible to apply for ILR:

If your UK residence visa has been granted on the basis of employment (either through a tier-1/2 visa, a sponsored visa or a combination thereof), then you need continuous unbroken residence status for five years before being eligible to apply for ILR. The period was only four years until April 2006.

If you are married/CPed to a British/EEA national, you need continuous unbroken residence status for two years before being eligible to apply for ILR on the basis of your relationship. To be eligible for ILR in this case, you need to be resident in the UK for a certain number of months in each year (normally nine out of every twelve-month block).

If you are the unmarried partner of a British/EEA national or a "settled" person (e.g., a non-British/EEA national but with ILR or Right of Abode) and legally resident in the UK through any means (e.g., as a student), then you are eligible to apply for FLR after two years of living with your partner and then ILR after another two years. To be eligible for FLR and ILR in these cases, you need to be resident in the UK for a certain number of months in each year (normally nine out of every twelve-month block).

If you are the married/CPed/unmarried domestic partner of a British national living outside of the UK, you are eligible to apply for FLR after two years of living with your partner anywhere in the world, and you are eligible to apply for ILR after four years of living with your partner anywhere in the world. (This law *may* have changed recently but I don't think so.)

That's all I can think of for now but others might know other circumstances which I have missed.