You can attach images. This way the image is uploaded to our server and automatically included in your post. There are restrictions for this as far as file size and format.
Another way is to upload the image to a webserver (for example, the one from your internet access provider). The link to that image (often something like http://www.internetprovider.com/username/image.jpg) is then used with the image tags ( [ Img]) in this forum. This way, the image is not stored on our server, but will be visible in the post still.
URL is short for the link to the website (Uniform Resource Locator), for example www.cdfreaks.com