USUL'I DIN & FURU'I DIN
The word usul is the plural of asl, which means a root or a principle, and furu is the plural of far which means a branch. The former is also called aqaid (pl. of aqida, lit. what one is bound to) or belief, and the latter ahkam (pl. of hukm, lit. an order) or the ordinances and regulations of Islam.
Roots are the parts of a tree, which thrive above the earth, and are seen. Those articles of the religion, which are concerned with the practice of it, and which are visible, are called the furu (branches). Mere roots without the branches are not called a complete tree, nor they can ever thrive underground, and the existence of such hidden thing without the proof of its being there, without any manifestation above the earth, can neither be believed in, nor can they be logically proved. Similarly, the usul'i din can neither remain safe, nor can be believed to be genuine in their existence, and nor they can be reasonably asserted to exist without the furu'i din, which are nothing but the evidential implementation of the usul. In short, as a tree is complete, prosperous and green, only when it has its roots as well as branches.
The religion of Islam may be divided into two parts