It is an operating system like Microsoft Windows, version X (10). Made by Apple.
Currently, it's $129 dollars (Mac OS X Leopard). Once Snow Leopard is available (August 28), it will be a $30 dollar upgrade from Mac OS X Leopard.
Mac OS X 10.6 (Snow Leopard) and all earlier versions of Mac OS X will support networks.
Mac OS X 10.5 is known as Leopard. The last available version of Leopard was 10.5.8.
To upgrade from Mac OS X 10.4 (Tiger) to 10.5 (Leopard) you will need to buy a boxed version of Mac OS X 10.5. If you have a Mac with an Intel processor you can go straight to Mac OS X 10.6 (Snow Leopard) which is a cheaper upgrade option.
You need to purchase each version (Panther/Tiger/Leopard) of Mac OS X.Mac OS X 10.3 (Panther) can be updated to 10.3.9.Mac OS X 10.4 (Tiger) can be updated to 10.4.11Mac OS X 10.5 (Leopard) can be updated to 10.5.8
Mac OS X 10.5 (Leopard)
Apple's Mac OS X. OS X Lion OS X Panther OS X Leopard etc.
Although the appearance of Mac OS X 10.5 (Leopard) and Mac OS X 10.6 (Snow Leopard) is similar there are many differences in the workings of the two systems. Most notably Snow Leopard requires a Mac with an Intel processor whereas Leopard will run on earlier Macs with G4 or G5 Power PC processor as well as the newer Intel based Macs.
Mac OS X 10.4 (Tiger) can be updated to Mac OS X 10.4.11 for free. Purchased upgrades then are Mac OS X 10.5 (Leopard) or Mac OS X 10.6 (Snow Leopard). Snow Leopard only works on Macs with an Intel processor. Older Macs with a G4 or G5 processor can only go up to Leopard.
Yes, Mac OS X 10.5 Leopard ships with Java support.
Mac OS X 10.4 was called Tiger and Mac OS X 10.5 was called Leopard. The 10.5 (Leopard) version is better than the 10.4 (Tiger) version. They would not have sold many copies of Leopard if it was worse than Tiger.
If you can download OS X from the internet, it's illegal.