Although Farseer Physics has origins in Box2D, it is not an exact port of it. It has some similarities but does things it's own way (more familiar with C# programmers I guess). It doesn't use Def classes. I believe the following 2 examples are equivalent:
C++
// create a body
b2BodyDef bodyDef;
bodyDef.type = b2_dynamicBody;
bodyDef.position.Set(-10, 20);
b2Body* body = world->CreateBody(&bodyDef);
// create a shape
b2CircleShape circleShape;
circleShape.m_radius = 0.5f;
// create a fixture
b2FixtureDef myFixtureDef;
myFixtureDef.shape = &circleShape;
body->CreateFixture(&myFixtureDef);
C#
// create a body
var body = world.CreateBody();
body.BodyType = BodyType.Dynamic;
body.Position = new Vector2(-10, 20);
// create a shape
var circleShape = new CircleShape(0.5f);
// create a fixture
var fixture = body.CreateFixture(circleShape);
As you can see from the body, the main difference is that you set the properties after creation on the actual instance rather than a def class.
Farseer Physics also makes use of handy Factory classes. Take a look at the documentation for more information.
https://farseerphysics.codeplex.com/documentation