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In Photoshop, when you put in some text, it stores it as text - ie if you type the word "hello", it stores the word "hello", and all the fonts and effects you chose. When you make some kind of picture, it's just stored as a big mass of coloured pixels. When you want to edit text as if it were a picture (eg chopping off half a letter or something), you need to convert it to a normal pixel-based layer. This is what rasterizing is. However, once you've rasterized a layer, Photoshop can't read it as text any more, so you can't text-edit it.