1. Al-Fara’id (pl. of fard or faridah) means any ordinance commanded by Allah or His Messenger. It is obligatory. Sometimes what is fard is a rukn. For example, washing face in wudu’ is obligatory (fard). It is a rukn too. These two terms are replacing each other in many contexts.
2. A rukn (trans. as pillar, pl. arkan) means what is inevitable or important of an entity by which the entity is there and it is perfect or valid. If one is missing, then there is incompletion or imperfection or invalidity.
3. When we say there are Five Pillars of Islam (arkan) it means that Islam is founded on those Five Pillars like a building, if one of those Pillars falls then the whole building will collapse.
It is stated in the Kuwaiti Encyclopedia of Jurisprudence: “Technically, rukn (or pillar) means what is inevitable or important of an entity and if missed the whole entity is no more existent or valid.
A fard technically means that which Allah strictly requires to be done. This is the same definition of wajib according to the majority of scholars. But the Hanafi scholars differntiates between the two defining the fard as that whose obligatory character is proved by definitive evidence, whereas the wajib is that which has been proved by speculative evidence.
The jurists are accustomed to use the word fard as a substitute to rukn, e.g. fara’id al-wudu’ (pillars or obligatory parts of wudu’), fara’id as-Salah (meaning the pillars or obligatory parts of prayer), etc.”
To conclude, in the juristic language there is no differnce between fard and rukn and they are used as substitutes to one another.