In TypeScript, an interface defines the structure of an object. It specifies the names and types of properties that an object must have. To define an interface, you use the `interface` keyword, followed by the interface name and the property definitions enclosed in braces `{ }`. Here's an example: 

interface Person {
  firstName: string;
  lastName: string;
  age: number;
  email?: string;
}
In this example, we define an interface called `Person` that has four properties: `firstName` and `lastName`, both of type `string`, `age` of type `number`, and an optional property `email` of type `string`. To use an interface, you can define an object that conforms to its structure: 

let person: Person = {
  firstName: "John",
  lastName: "Doe",
  age: 30,
  email: "john.doe@example.com"
};
Here, we define an object `person` that conforms to the `Person` interface. It has properties `firstName`, `lastName`, `age`, and `email`. If an object doesn't have all the required properties defined in an interface, TypeScript will generate an error: 

let person: Person = {
  firstName: "John",
  age: 30 // Error: Property 'lastName' is missing
};
You can also use interfaces as types for function parameters and return values: 

function getFullName(person: Person): string {
  return `${person.firstName} ${person.lastName}`;
}
Here, the `getFullName` function takes a parameter of type `Person` and returns a string. Interfaces can also extend other interfaces and define optional and readonly properties, among other things.