Lesson:Background Eraser Tool
When a picture file (.jpeg, .PNG, .GIF, .BMP or any image file format) is opened in Photoshop, the layer palette will display it as the back ground layer. If you try to erase part of the image, the selected background colour will be filled to that area. That means you won't get transparent background
But with the Background Erase tool you can erase the background layer & the erased portion will be transparent!