If you want to be the greatest Pokemon trainer there ever was, you should know the strengths and weaknesses of not only your own Pokemon, but also of other Pokemon that you want to fight or tame. This cheatsheet from Pokemon Go Database lets you quickly reference how your own Pokemon measure up against others.
In every Pokemon game, attacks from one Pokemon type to another can have a large, normal, or no effect. It all depends on the match-up. For example, Squirtle is a water-type Pokemon, so attacks on a fire-type like Charmander are very effective. If you've played the older Pokemon games, you might find that some mechanics work a bit differently in Pokemon Go.
You can find out your Pokemon type by checking your Pokedex. Check the chart below to see the amount of damage that can be dealt or received, based on the Pokemon match-ups.
[Via Pokemon Go Database]