The first important item to be concerned about is attack range, or the distance from the attacker to the target.
This is measured in grid squares and diagonal squares are considered to be the same distance as adjacent squares. Firing four squares diagonally is a lot more distance than firing four squares straight forwards, if you measure with a ruler, but in terms of the game this doesn't matter and it is still just four squares either way.
Each weapon has a maximum distance listed in its description. For most missiles, this is "the entire screen". Missiles fired from a ship can be shot from anywhere and they will travel till they hit the target or until two turns have passed and they have not hit the target. If they go that long without hitting the target they self destruct and do nothing. Missiles travel a certain number of squares per turn, listed in the description, and it is entirely possible to outrun them as a strategy.
It is probably not a good one most of the time, but the computer can do it and do it well.
Missiles fired from planetary missile bases have much greater fuel levels. Some sources say they can travel for four turns, others say they never run out of fuel. The first is probably right, but this pretty much never impacts gameplay either way. If the target can avoid the missile for four turns it can probably avoid it indefinitely.
Missiles and Torpedoes are mostly the same except that torpedoes have an unlimited number of shots but can only fire every second turn whereas missiles (on a ship) run out of ammo after two or five shots.
Generally missiles are better because most space combats are not long enough to run the attacker out of a five shot rack of ammunition, and most are already decided by the time the second shot from a two shot rack hits the target. With this being the normal case, it is better to shoot every turn than every second turn. If you are designing a "Juggernaut" ship, or one that is meant to be invincible and just stay alive chipping away at the enemy slowly over time the torpedoes could be better. It is hard to make this kind of ship work, though, so regular missiles are still usually better.
The special equipment High Energy Focus adds three to the range of all weapons on the equipped ship that are not missiles, torpedoes, or bombs. More than any other technology, this technology has the ability to ensure that the possessor dominates interstellar combat. If there is an "auto win" tech, this is probably it.