What is the difference between a weak reference and an unowned reference?

Both weak and unowned references do not create a strong hold on the referred object (a.k.a. they don't increase the retain count in order to prevent ARC from deallocating the referred object).

But why two keywords? This distinction has to do with the fact that Optional types are built-in the Swift language. Long story short about them: optional types offer memory safety (this works beautifully with Swift's constructor rules - which are strict in order to provide this benefit).

A weak reference allows the possibility of it to become nil (this happens automatically when the referenced object is deallocated), therefore the type of your property must be optional - so you, as a programmer, are obligated to check it before you use it (basically the compiler forces you, as much as it can, to write safe code).

An unowned reference presumes that it will never become nil during its lifetime. An unowned reference must be set during initialization - this means that the reference will be defined as a non-optional type that can be used safely without checks. If somehow the object being referred to is deallocated, then the app will crash when the unowned reference is used.

From the Apple docs:

Use a weak reference whenever it is valid for that reference to become nil at some point during its lifetime. Conversely, use an unowned reference when you know that the reference will never be nil once it has been set during initialization.

In the docs, there are some examples that discuss retain cycles and how to break them. All these examples are extracted from the docs.

Example of the weak keyword:

class Person {
    let name: String
    init(name: String) { self.name = name }
    var apartment: Apartment?
}
 
class Apartment {
    let number: Int
    init(number: Int) { self.number = number }
    weak var tenant: Person?
}

And now, for some ASCII art (you should go see the docs - they have pretty diagrams):

Person ===(strong)==> Apartment
Person <==(weak)===== Apartment

The Person and Apartment example shows a situation where two properties, both of which are allowed to be nil, have the potential to cause a strong reference cycle. This scenario is best resolved with a weak reference. Both entities can exist without having a strict dependency upon the other.

Example of the unowned keyword:

class Customer {
    let name: String
    var card: CreditCard?
    init(name: String) { self.name = name }
}
 
class CreditCard {
    let number: UInt64
    unowned let customer: Customer
    init(number: UInt64, customer: Customer) { self.number = number; self.customer = customer }
}

In this example, a Customer may or may not have a CreditCard, but a CreditCard will always be associated with a Customer. To represent this, the Customer class has an optional card property, but the CreditCard class has a non-optional (and unowned) customer property.

Customer ===(strong)==> CreditCard
Customer <==(unowned)== CreditCard

The Customer and CreditCard example shows a situation where one property that is allowed to be nil and another property that cannot be nil has the potential to cause a strong reference cycle. This scenario is best resolved with an unowned reference.

Note from Apple:

Weak references must be declared as variables, to indicate that their value can change at runtime. A weak reference cannot be declared as a constant.

There is also a third scenario when both properties should always have a value, and neither property should ever be nil once initialization is complete.

And there are also the classic retain cycle scenarios to avoid when working with closures.

For this, I encourage you to visit the Apple docs, or read the book.

Q1. How is an “Unowned reference” different from a “Weak Reference”?

Weak Reference:

A weak reference is a reference that does not keep a strong hold on the instance it refers to, and so does not stop ARC from disposing of the referenced instance. Because weak references are allowed to have “no value”, you must declare every weak reference as having an optional type. (Apple Docs)

Unowned Reference:

Like weak references, an unowned reference does not keep a strong hold on the instance it refers to. Unlike a weak reference, however, an unowned reference is assumed to always have a value. Because of this, an unowned reference is always defined as a non-optional type. (Apple Docs)

When to Use Each:

Use a weak reference whenever it is valid for that reference to become nil at some point during its lifetime. Conversely, use an unowned reference when you know that the reference will never be nil once it has been set during initialization. (Apple Docs)

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Q2. When is it safe to use an “unowned reference”?

As quoted above, an unowned reference is assumed to always have a value. So you should only use it when you are sure that the reference will never be nil. Apple Docs illustrate a use-case for unowned references through the following example.

Suppose we have two classes Customer and CreditCard. A customer can exist without a credit card, but a credit card will not exist without a customer, i.e. it can be assumed that a credit card will always have a customer. So, they should have the following relationship:

class Customer {
    var card: CreditCard?
}

class CreditCard {
    unowned let customer: Customer
}

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Q3. Are “unowned reference” reference an security risk like “dangling pointers” in C/C++

I don't think so.

Since unowned references are just weak references that are guaranteed to have a value, it shouldn't be a security risk in any way. However, if you try to access an unowned reference after the instance that it references is deallocated, you will trigger a runtime error, and the app will crash.

That's the only risk I see with it.

Link to Apple Docs