A custom virtual machine simply means a virtual machine that you create using the From Gallery option because it gives you more configuration choices than the Quick Create option. These choices include:
If you want your virtual machine to use a virtual network so you can connect to it directly by host name or set up cross-premises connections, make sure that you specify the virtual network when you create the virtual machine. A virtual machine can be configured to join a virtual network only when you create the virtual machine. For details on virtual networks, see Azure Virtual Network overview.
On the command bar at the bottom of the window, click New.
Under Compute, click Virtual Machine, and then click From Gallery.
The first screen after this lets you Choose an Image for your virtual machine from the list of available images. (The available images may differ depending on the subscription you're using.)
The second screen lets you pick a computer name, size, and administrative user name and password. Use the tier and size required to run your app or workload. Here are some tips:
The third screen lets you configure resources for networking, storage, and availability. Here are some tips:
If you want a virtual machine to use a virtual network, you must specify the virtual network when you create the virtual machine. You can't join the virtual machine to a virtual network after you create the VM. For more information, see Azure Virtual Network Overview.
For details about configuring endpoints, see How to Set Up Endpoints to a Virtual Machine.
The fourth configuration screen lets you install the VM Agent and configure some of the available extensions.
The VM agent provides the environment for you to install extensions that can help you interact with or manage the virtual machine. For details, see About the VM agent and extensions.
After the virtual machine is created, the portal lists the new virtual machine under Virtual Machines. The corresponding cloud service and storage account also are created and are listed in those sections. Both the virtual machine and cloud service are started automatically and their status is listed as Running.