Career as a noun is used to describe the job or series of jobs you do during your working life, especially if you continue to get better jobs and earn more money.
For example: "I never imagined I would end up with a career in teaching."
A carer is a family member or paid helper who regularly looks after a child or a sick, elderly, or disabled person.
For example: " Some people start being carers at a very young age."
Carrier is a noun, it can mean person or thing that carries something, or someone who does not suffer from a disease but has the infection can give it to someone else.
For example: "Australians voted British Airways, their least favourite carrier for flights to the UK."
A courier is someone who carries important messages, packages or documents for someone else.
For example: "The United Parcel Service (UPS) is the largest courier service in the world."