Thank you for your question. An infallible is somebody who is protected from committing sin as their knowledge and faith are such that it would be completely irrational (and therefore impossible) for them to commit any kind of sin. Their purification is to such an extent that their commanding soul does not call them to sin as it is completely under the control of their intellect.
An infallible is a sinless person, but being sinless is more general and could also include someone who hasn't committed a sin yet but perhaps doesn't have the knowledge and faith to make them infallible. The two are therefore the same in that they haven't committed sin, but can also be different from the perspective of knowledge and faith. An infallible is sinless and always will be.
May you always be successful.