In Android, apps like Gallery and Music Player will automatically add all the medias that are in your phone. Though this is a wonderful feature sometimes this can cause problem. For eg: I used one program to download some podcast. All the images that this program downloaded are now also visible in Gallery. This isn't what I want. I don't want to see some tiny unrelated web images downloaded by podcast app in Gallery.
In a way this is developers fault. This can be easily prevented by creating a .nomedia file.
If there is a file called .nomedia in any folders, then any app that auto scans for media will ignore that media. So the developers of app that puts media in you SD card, where the media shouldn't pop up in apps like Gallery need to create .nomedia file in that folder.
If the developer hasn't done so. We can easily create it. We can use any file manager program like ES File Explorer or Astro File Manager. With such program navigate to the folder and choose New File and give the file name .nomedia . The file can be a empty file.
Now media auto scanning apps like Gallery or Music will ignore that folder.