Fewer is an adjective.
Meaning: a smaller quantity in terms of number (of things)
Less is an adjective.
Meaning: a smaller quantity in terms of amount (of something)
Note: both fewer and less can be used in one sentence.
Examples of fewer and less being used together in a sentence:
1. Fewer the people involved in the project, the less likelihood of details being leaked out.
2. The fewer the security personnel, the lesser possibility of managing the crowds.
Examples of fewer:
1. Fewer people attended the conference than was expected by the organisers.
2. Fewer and fewer birds flock to the lake each year.
Examples of less:
1. A lesser amount of eggs would have made the cake tastier.
2. A lesser number of red flowers will give the bouquet a pretty look.