What are abstract classes and interfaces in TypeScript?

Abstract classes and interfaces in TypeScript are powerful features that support object-oriented programming (OOP).

1. Abstract Classes: Abstract classes are base classes from which other classes may be derived. They may not be instantiated directly. Unlike an interface, an abstract class may contain implementation details for its members. The abstract keyword is used to define abstract classes as well as abstract methods within an abstract class.

Example:

abstract class Animal {
    abstract makeSound(): void;

    move(): void {
        console.log('Moving along!');
    }
}

In the above example, Animal is an abstract class that has an abstract method makeSound. It also has a normal method move.

2. Interfaces: Interfaces are a powerful way to define contracts within your code and they are the bread and butter of software design in TypeScript. They are used to type-check whether an object fits a certain structure. By defining an interface we can name a specific combination of variables, making sure that they will always go together.

Example:

interface LabelledValue {
    label: string;
}

function printLabel(labelledObj: LabelledValue) {
    console.log(labelledObj.label);
}

let myObj = {size: 10, label: "Size 10 Object"};
printLabel(myObj);

In the above example, LabelledValue is an interface and it's used to type-check myObj. If myObj doesn't have a label property, TypeScript will throw an error.

In summary, abstract classes and interfaces are two of the ways TypeScript allows you to write code in a more object-oriented style. Abstract classes provide a way to define classes that can't be instantiated, but can be extended. Interfaces provide a way to define the shape of certain data.