There are two ways in which digital images can be compressed: lossless and lossy.
Lossless compression is similar to what WinZip does. For example, if you compress a document into a ZIP file and later extract and open the document, the content will be identical to the original. No information is lost in the process. Only some processing time was required to compress and decompress the document. TIFF is an image format that can be compressed in a lossless way.
Lossy compression reduces the image size by discarding information and is similar to summarizing a document. For instance, you can summarize a 100 page document into a 75 page or 20 page document that represents the original, but you cannot create the original out of the summary as information was discarded during summarization. JPEG is an image format that is based on lossy compression.