What is the main difference between flagella and pili?

Hint: Pili and flagella have important roles on the bacterial cell surface, such as motility, adhesion, protein and DNA absorption through the horizontal gene transfer mechanisms particularly conjugation. These structures may expand to many times the size of a cell, resist and create tremendous pressures, and quickly form and deconstruct in response to environmental cues. Pili and flagella are both protein-based filamentous structures.

Complete answer:
Not all but many bacteria have pili and flagella on their surfaces and since both are filamentous in nature they are often confused to be the same, although they have significant differences between each other.
The major points of differences between flagella and pili are as follows -


Note:
Bacteria and Archaea have a variety of thread-like proteinaceous organelles on their exterior surfaces with which they interact with their surroundings. Pili, flagella, and other filamentous polymers are composed of repeated assemblies of covalently or noncovalently linked protein components. Flagella are known to be found in the cells of both prokaryotes and eukaryotes.