Differences between pili and fimbriae are:
Pili :
They are tube like outgrowths of bacterial cells.
Their length is about 18-20 pm.
Only 1-4 pili occur per cell.
They help in conjugation.
Fimbriae :
They are solid outgrowths of bacterial cells.
Their length is about 0.15-1.5pm.
About 300-400 fimbriae occur per cell.
They help in clinging to various objects.