github.com/guilhermebr/docker@v1.4.2-0.20150428121140-67da055cebca/docs/sources/terms/image.md (about)

     1  page_title: Image
     2  page_description: Definition of an image
     3  page_keywords: containers, lxc, concepts, explanation, image, container
     4  
     5  # Image
     6  
     7  ## Introduction
     8  
     9  ![](/terms/images/docker-filesystems-debian.png)
    10  
    11  In Docker terminology, a read-only [*Layer*](/terms/layer/#layer) is
    12  called an **image**. An image never changes.
    13  
    14  Since Docker uses a [*Union File System*](/terms/layer/#union-file-system), the
    15  processes think the whole file system is mounted read-write. But all the
    16  changes go to the top-most writeable layer, and underneath, the original
    17  file in the read-only image is unchanged. Since images don't change,
    18  images do not have state.
    19  
    20  ![](/terms/images/docker-filesystems-debianrw.png)
    21  
    22  ## Parent image
    23  
    24  ![](/terms/images/docker-filesystems-multilayer.png)
    25  
    26  Each image may depend on one more image which forms the layer beneath
    27  it. We sometimes say that the lower image is the **parent** of the upper
    28  image.
    29  
    30  ## Base image
    31  
    32  An image that has no parent is a **base image**.
    33  
    34  ## Image IDs
    35  
    36  All images are identified by a 64 hexadecimal digit string (internally a
    37  256bit value). To simplify their use, a short ID of the first 12
    38  characters can be used on the command line. There is a small possibility
    39  of short id collisions, so the docker server will always return the long
    40  ID.