PSD Files
These are propriety files used specifically with Adobe Photoshop. Photoshop saves files as this by default, and once opened again the image is in the same condition in Photoshop as it was before, including all layers and effects. This allows it to be ready to edit. The only drawback to this is they have very large file sizes. Photoshop gives the option to compress these files and save them as non-propriety formats such as JPG, GIF, TIFF, etc. however they cannot be reverted back.
Source http://whatis.techtarget.com/fileformat/PSD-Adobe-Photoshop-default
JPEG Files
A JPEG is a compressed image file format with no limit on the colours that can be used like a GIF file has. This has lead it to being the most common type of image file used around the internet. JPEG stands for Joint Photographic Experts Group, after the committee that created the file type. Although JPEGs can create colourful, high quality images, they are lossy file types, meaning they lose quality every time the image is compressed. This may not be noticeable until the file has been compressed a lot of times, then it can end up looking blocky and low quality.
Source http://pc.net/glossary/definition/jpeg
Below is an image that shows what happens as the file is compressed more;
http://www.exposureguide.com/images/file-formats/jpeg-compression.jpg
BMP Files
BMP is short for Bitmap. It is a raster image file type that originated on the Windows operating system. Colour is stored for each pixel without compression, meaning although high quality images can be produced, file sizes are often very large. File types such as JPEG are also Bitmaps, but they use compression algorithms to decrease their file sizes. This means that files such as those are used on the internet, while the BMP file type is used for images that are to be printed.
Source http://www.techterms.com/definition/bmp
PNG Files
PNG stands for Portable Network Graphic, and it was developed by the PNG Development Group and released in 1996. It was designed to be a patent free alternative to the GIF format and is expected to replace it as it improve on many of the GIF formats features, such as better compression and smaller file sizes. It is a raster format, and it is lossless meaning it does not lose quality when compressed and decompressed. It supports pallet based images, grayscale images, and full colour non-palette images however as it was designed for online image sharing rather than professional use; it does not support non-RGB color spaces.
Source http://en.wikipedia.org/wiki/Portable_Network_Graphics
The image below shows comparison between a JPEG and a PNG
http://upload.wikimedia.org/wikipedia/commons/a/a4/Comparison_of_JPEG_and_PNG.png
TIFF Files
TIFF stands for Tagged Image File Format. It is a Raster/Bitmap file type and produces high quality images. It was created in 1986 by Aldus Corporation with the intention of it being the sole file type used by companies for black and white scanned images. It was later given a color pallet and nowadays is used by graphic artists for high color depth images. It is supported by almost all image manipulation software. TIFF files make use of a lossless data compression technique called LZW compression.
http://whatis.techtarget.com/fileformat/TIF-Tag-image-bitmap-file-TIFF
GIF Files
A GIF, or Graphic Interchange File format, is a compressed image format. The compression formula it uses was created by CompuServe. They are based on index colours, meaning a GIF can support a colour pallet of 256 colors. This results in a small file size, meaning they are often used across the Internet as they can be sent easily. They make great icons and animations but since they have a limited colour palette, they are unable to be used for large, high quality images.
http://www.techterms.com/definition/gif
http://www.practicalecommerce.com/files/images/0001/0138/file_type_comparison.png
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