Tepal is one of the component parts of the perianth, the outermost whorls of flower parts, especially when the perianth is not divided into two whorls of unequal appearance while perianth is the sterile parts of a flower; collectively, the sepals and petals (or tepals).

Hi! First of all. You should be very clear with the meaning of perianth. Perianth is a term used for corolla and calyx when not distinct according to class 11th ncert but! Higher books suggest Perianth to be just a collective term for corolla and calyx regardless of the fact whether they are distinct or not!...now moving back to the main stream question. What makes tepals different from perianth as a *term* 

Tepals are part of perianth. The only thing to be clear of is that the plant families have their flower having multiples of whorls and in the cases of perianth ( class - liliaceace considering class 11) Liliaceace will have multiple layers of perianth in which the OUTERMOST unit will be termed as TEPALS. Hence; tepals are also considered to be the protection unit of buds as it is the OUTERMOST layer of perianth.

Hope the Answer is clear to the readers

~Dr.Aarohi Rajwar