Who is an accomplice? Can a court pass order of conviction on the basis of statement made by an accomplice?

A person who knowingly, voluntarily, or intentionally gives assistance to another in (or in some cases fails to prevent another from) the commission of a crime. An accomplice is criminally liable to the same extent as the principal. An accomplice, unlike an accessory, is typically present when the crime is committed. 

According to section 133 an accomplice shall be a competent witness against an accused person; and a conviction is not illegal merely because it proceeds upon the uncorroborated testimony of an accomplice.

Accomplice must have 

1 Participate in crime 
2 Asserts in the offence 
3 Abet an offence

Thus, the provision of section 133 show that 
(a) Accomplice can be a competent witness against the accused 
(b) A conviction is not illegal merely because it proceeds upon the uncorroborated testimony of an accomplice. 

Whether corroboration is required? 

Although, according to section 113, the conviction of an accused does not require corroboration of the testimony of an accomplice, but it is seen in the context of section 114 

Why Corroboration is required? 

It is an important question that why the corroboration of the testimony of the accomplice is necessary. Here are the reasons why it is necessary. 
(a) The accomplice is criminal himself, and therefore, his testimony should not carry the same respect as that of a law abiding citizen. 
(b) He has been faithless to his companions and may be faithless to the court because he has motive to shift the guilt from himself to his former companions 
(c) If he is an approver he has been favoured by the state and is, therefore, likely to favour the state.