There are many tape formats. The two most popular are LTO and DLT below is some info about the format and Links to more info regarding capacities and compatibility.
LTO : Linear Tape-Open (LTO) technology was developed jointly by HP, IBM and Certance to provide a clear and viable choice in an increasingly complex array of tape storage options. LTO technology is an "open format" technology, which means that users will have multiple sources of product and media. The "open" nature of LTO technology also provides a means of enabling compatibility between different vendors' offerings.
The Ultrium format is the "high capacity" implementation of LTO technology.
DLT :DLT tape Technology is the most trusted and compatible tape platform available and is the choice of leading brands worldwide. Brands like ADIC, Compaq, Dell, Exabyte, Fuji, HP, IBM, Legato, Maxell, Network Appliance, Overland Storage, Quantum, StorageTek, Sun Microsystems, Tandberg and Veritas Software. In fact, 98% of FORTUNE 500® companies trust DLTtape Technology to back up their business critical data every day. With over 2 million drives and over 100 million cartridges shipped worldwide, you simply can't find a more compatible family of tape products anywhere.