Pictures may be stored on the computer in a number of different formats. The different formats are different ways the pictures are stored in the computer code and different programs can work with different formats. Some formats compress the file storing the picture making it take less space on your disk, but the quality can decline.
The most common picture format is jpg or jpeg. You usually see it use the first extension (jpg). Almost all programs and web browsers use jpg and most cameras shoot in jpeg format by default and many cameras that is the only format they use.
Raw mode is used by many better quality cameras and profession photographers as it does not automatically compress or do some settings. On a Canon camera the raw mode extension is crw. The pictures take a lot more space but are better quality. Programs like Photoshop can convert raw mode pictures to jpg format.
If you do drawings in Paint and save the images quite often it will save them as gif or bmp which are not as good of quality. Png was introduced as a format that would be better quality pictures for the Internet but has never really caught on although the web browsers all recognize them and you will find used sometimes.
Generally jpg is the best format to use for your pictures and choose to use the best resolution so they will be sharper and better.
The file format (extension) name can use upper or lower case letters and is treated as same by the computer as recognizing the type. On Linux/Unix computers (which are used for some web site hosting) can see a picture with lower case as different from upper case so dwight.jpg is a different picture than dwight.JPG however on Windows machines it would allow only one name as Windows is not case sensitive.