A conjecture is a statement that you believe to be true, and that has be verified by your observations, not necessarily exhaustively. That is to say, your conjecture may not even be true, although you believe that it is.
If we denote by S the sum of the first 30 positive even numbers, we could make many conjectures, such as:
S is even
S is positive,
S=930
S is composite
.... etc.