what is meant by Divide by N and Modulo(N) in counters?
A Divide by N counter implies that it  divides the input clock frequency by N ie; if you cascade four flip-flops then, the output of every stage is divided by 2, if you are taking the output from the 4th flip-flop, then its output frequency is clock frequency by 16 (2^4).

Example: take a look at the timing diagram of a 3bit counter



A modulo N counter does the same thing as above and is also used as an alternate definition which implies that the counter counts N states.

Example: If a MOD-10 counter counts 10 states ( 0000 - 1000 )