Permanent residency or Indefinite leave to remain (ILR) is an immigration status that allows a person to reside for indefinite time in the UK. The person who receives ILR is free to live in the country without renewal of the visa. After ILR visa status, the person is free to work without restrictions. He also qualifies for EU or EEA tuition rates at any UK university and can apply for loans. However, the person with an Indefinite Leave to Remain status is not a citizen of Great Britain. He can apply for Naturalisation status or Citizenship after one year of receiving ILR.
You must understand that ILR is not a permanent status and can lapse if you remain outside the country for continuous 2 years. To keep your ILR status you have to spend more time in the Britain then elsewhere. If your nature of work demands more travel and overseas stays, it is better to apply for UK citizenship as soon as you are eligible for it.
You cannot apply for ILR directly. First, you have to migrate to the UK with any of the visas available for non-EU and EEA. The time in each visa category is different. For example, you should spend 5 years in the UK with Tier 1 or Tier 2 visa before applying for permanent residency using FORM SET (O). Similarly, retired persons of independant menas use FORM SET (BUS).
As there are numerous clause and forms, you must speak to a registered agent or consultant to know which form is applicable for you.