User Control vs. Windows Form

Solution 1:

Put very simply:

User controls are a way of making a custom, reusable component. A user control can contain other controls but must be hosted by a form.

Windows forms are the container for controls, including user controls. While it contains many similar attributes as a user control, it's primary purpose is to host controls.

Solution 2:

They have a lot in common, they are both derived from ContainerControl. UserControl however is designed to be a child window, it needs to be placed in a container. Form was designed to be a top-level window without a parent.

You can actually turn a Form into a child window by setting its TopLevel property to false:

public partial class Form1 : Form {
    public Form1() {
        InitializeComponent();
        var child = new Form2();
        child.TopLevel = false;
        child.Location = new Point(10, 5);
        child.Size = new Size(100, 100);
        child.BackColor = Color.Yellow;
        child.FormBorderStyle = FormBorderStyle.None;
        child.Visible = true;
        this.Controls.Add(child);
    }
}