Tippet
Tippet is a spool of fine, clear material that is either a Monofilament, most generally nylon or, Fluorocarbon. “Mono” (short for monofilament) floats, is ideal for dry flies and is a bit stretchier than “fluoro” (short for fluorocarbon). Fluoro is a denser material and sinks. It becomes almost invisible in water and is highly abrasion resistant. It is better to use when nymphing or fishing streamers or with subsurface fly patterns.
Tippet is categorized by an X Rating system. The higher the number, the smaller in diameter, finer and weaker the material. The X Rating system goes from 8x (finest, weakest, thinnest) to Ox (thickest and strongest).
Tippet material comes in spools and is the same thickness/diameter throughout the entire spool of material. It is level. It is used as the final end terminal that you will tie your fly onto.
Leader
On the other hand, a Leader is a tapered piece of monofliament or fluorocarbon line that attached at the thickest, heaviest end, to your fly line and then gradually tappers down to a much finer opposite end where you will attach your fly.
Because regular fly line is thicker than tenkara line, a leader is used to create an energy transition section to allow the fly to land more delicately than if straight tippet were tied onto the end of a thicker, heavier fly line. A leader consists of 3 parts: the Butt Section where you attach it to your fly line. This is the thickest, heaviest part. The Tapered Section in the middle gradually decreases in thickness and helps the line unroll smoothly. And finally, the Tippet Section, which is the thinnest, final section where you tie your fly on.
In traditional tenkara setups, anglers use fine, short tenkara lines that are ultralight. A tapered transition section (a leader) is less or unnecessary for proper line turnover, unfolding and delicate fly presentation. Can you use a Tapered Leader with your tenkara rod and line? Yes. But it generally isn’t necessary unless you are using a nontraditional tenkara setup with a long or heavier line. Hope this makes sense and clears things up for you.