Updated for Swift 5.1

Assume the following throwing function:

enum ThrowableError: Error {

    case badError(howBad: Int)
}

func doSomething(everythingIsFine: Bool = false) throws -> String {

  if everythingIsFine {
      return "Everything is ok"
  } else {
      throw ThrowableError.badError(howBad: 4)
  }
}

try

You have 2 options when you try calling a function that may throw.

You can take responsibility of handling errors by surrounding your call within a do-catch block:

do {
    let result = try doSomething()
}
catch ThrowableError.badError(let howBad) {
    // Here you know about the error
    // Feel free to handle or to re-throw

    // 1. Handle
    print("Bad Error (How Bad Level: \(howBad)")

    // 2. Re-throw
    throw ThrowableError.badError(howBad: howBad)
}

Or just try calling the function, and pass the error along to the next caller in the call chain:

func doSomeOtherThing() throws -> Void {    
    // Not within a do-catch block.
    // Any errors will be re-thrown to callers.
    let result = try doSomething()
}

try!

What happens when you try to access an implicitly unwrapped optional with a nil inside it? Yes, true, the app will CRASH!
Same goes with try! it basically ignores the error chain, and declares a “do or die” situation. If the called function didn’t throw any errors, everything goes fine. But if it failed and threw an error, your application will simply crash.

let result = try! doSomething() // if an error was thrown, CRASH!

try?

A new keyword that was introduced in Xcode 7 beta 6. It returns an optional that unwraps successful values, and catches error by returning nil.

if let result = try? doSomething() {
    // doSomething succeeded, and result is unwrapped.
} else {
    // Ouch, doSomething() threw an error.
}

Or we can use guard:

guard let result = try? doSomething() else {
    // Ouch, doSomething() threw an error.
}
// doSomething succeeded, and result is unwrapped.

One final note here, by using try? note that you’re discarding the error that took place, as it’s translated to a nil.
Use try? when you’re focusing more on successes and failure, not on why things failed.

Using Coalescing Operator ??

You can use the coalescing operator ?? with try? to provide a default value incase of failure:

let result = (try? doSomething()) ?? "Default Value"
print(result) // Default Value