This is a list of number theory topics, by Wikipedia page. See also list of cryptology topics.
NB Listing here is not perjorative, since many famous topics in number theory have origins in challenging problems posed purely for their own sake. See recreational mathematics.
Factors
Modular arithmetic
Primality tests and factorization
Arithmetic functions
Analytic number theory: additive problems
Algebraic number theory
Quadratic forms
L-functions
Diophantine equations
Diophantine approximation
Named primes
Pseudo-random numbers
Recreational topics