C4D stands for Cinema 4D, which is the program used to make them. They're used in signatures or in large pieces/whatever to add effects and flow and to make it look nice.
Here are some examples:
You can get them by searching in Google Images, or you can go to www.planetrenders.net -- they have some good ones there, or you can search and download free c4d packs from Google.
In Photoshop, you open up the c4d you want, drag it onto your signature (or whatever you're doing) and then set the layer to screen or lighten, and position until it looks good.
Hope I helped
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