To represent a vector geometrically, we use arrows (also known as "directed line segments"). The length of the arrow represents the vector's size, and its direction represents the vector's direction in 2-space or in 3-space.

For example , we could represent a displacement 2 km southwest by the arrow in the diagram.  

Vectors are parallel if their shafts are parallel. They must have either the same direction or opposite directions, but can have any length.

Though it has no direction, the zero vector is thought of as being parallel to every vector.  

Anatomy of a geometric vector

To describe a vector, we refer to its tail (more formally: its initial point), its head (more formally: its terminal point) and its shaft.

If its head and tail coincide, the vector has length 0, and is called the zero vector. It is usually not considered to have any direction, and is represented by a simple dot.
Labeling vectors. There are several common ways to label a vector.

Use a single letter with an arrow:

Boldface letters are easier to print than arrows, so they are used mainly in books.

It's difficult to write boldface with a pencil or pen, so the single letter with arrow is used more for handwritten materials like classroom notes.

The "head-point to tail-point" form is often used when the vector joins previously named points. However, it is difficult to reproduce this notation on a web page, so instead, we'll omit the arrow and just use boldface letters, like AB.

Use a single boldface letter:

Use the names of its head and tail with an arrow:

The zero vector is usually labeled by the number 0, i.e. as.
There's something important you should notice here: the arrow representing a vector quantity only needs to represent its magnitude and its direction, so the actual positions of its head and tail don't matter, only their positions relative to each other. Different arrows represent the same vector if they have the same direction and the same length.
If you change an arrow in the diagram by dragging its shaft, its head and tail stay in the same positions relative to each other, so you don't change the vector the arrow represents.

If you change an arrow by dragging its head or tail, you change the relation between the head and tail, so you change the vector the arrow represents.