.psd
Photoshop has ties with other Adobe Software for media editing, animation, and authoring. The .PSD (Photoshop Document), Photoshop's native format, stores an image with support for most imaging options available in Photoshop. These include layers with masks, color spaces, ICC profiles, transparency, text, alpha channels and spot colors, clipping paths, and duotone settings. This is in contrast to many other file formats (e.g. .EPS or .GIF) that restrict content to provide streamlined, predictable functionality.
Portable Document Format. It's like an un-modifiable text document. Photoshop can output to it.
Extensions are small programs which can run inside Photoshop. Example is Mini Bridge, Kuler in Photoshop CS5 but there are and many 3rd party extensions. Sometimes people refers to file format as extension or file extension like .csh extension for example to save Custom Shapes.
I think you mean PSD, which stands for Photo Shop Document. It is the name of the file extension created when you create or edit a multi-layered image in Photoshop.
almost any format:.jpg , .gif , .png are most common but you can open almost any other format
Adobe illustrator (AI) is a vector file format to save vector illustrations. It is the default format for Adobe illustrator. Photoshop cannot save the images in to AI format but it can be opened in Photoshop.
Photoshop files use the extension .psd in Mac OS X.
In Photoshop - PSD file format.
Gimp can open a .PSD file directly, and save it out as it's native format - the .XCF
Photoshop has ties with other Adobe software for media editing, animation, and authoring. The .PSD (Photoshop Document), Photoshop's native format, stores an image with support for most imaging options available in Photoshop. These include layers with masks, color spaces, ICC profiles, transparency, text, alpha channels and spot colors, clipping paths, and duotone settings. This is in contrast to many other file formats (e.g. .EPS or .GIF) that restrict content to provide streamlined, predictable functionality.
psd is a photoshop extension and can be opened with photoshop or photoshop elements.
You can open .gif file in Photoshop, to convert it to another format, first open it in Photoshop then go to Save As and choose another format to save. If you cannot open that file in photoshop then See related link for free online file conversion, convert to png, jpg or tiff and you will open file in Photoshop.
When a photoshop file is too large (in bytes) to save as a regular .psd, it will be saved as a Photoshop Large Document Format or a .psb file.