An example of an invalid argument is: "All ceilings are attached to walls. All doors are attached to walls. Therefore, all doors are ceilings." An example of a valid but unsound argument is: "All dogs are green. Anything that is green is a fish. Therefore, all dogs are fish."
For an argument to be valid, it must take a form where it is impossible for the premises to be true and the conclusion to be false. If the premises don't logically guarantee the conclusion, then the argument is invalid. Truth doesn't factor into whether an argument is valid or not. In fact, an argument might be perfectly valid but still have a false conclusion. This is where soundness comes in. If an argument is sound, then all of the premises are true. If the conclusion is true and the premises are not, then the argument is unsound.
All sound arguments are also valid arguments. However, while an argument can be invalid or valid but unsound, it doesn't mean that the conclusion is false. The conclusion might be perfectly true, but the person doing the arguing got there through incorrect means. A good way to know if an argument is invalid or unsound is to translate it into symbolic logic.