PlayStation is a popular brand of Video Games Console From Sony, it comes in four main varieties
The PlayStation, released in 1994 it was Sony's first video games console, it completely dominated the fifth generation of consoles seeing off competition from Sega's Saturn, only the N64 provided any real competition globally. Because it was so successful it had an incredibly long life span, which meant that developers had to increasingly push the limits of the consoles ability towards the end of its life till the point where some users thought the games might cause the console to explode, shortly before its replacement was released in 2001 Sony released the PSone a smaller version that could be somewhat used portability and appealed to the budget end of the gaming market, it was finally discontinued in 2006.
The PlayStation 2 was released in 2000, it managed to exceed the sales of its predecessor becoming the best selling console of all time . It is fully backwards compatible with original PlayStation games and even has the ability to improve their graphics, unlike the competition it could also play DVD's. The PlayStation 2 did have stronger competition than than the previous system from the Microsoft XBox which was slightly more powerful however it still outsold it 6-1, the only other competition was from the more budget orientated Nintendo GameCube. Even as of the end of 2008 it is still the most played console 
The PlayStation 3 was released late in 2006, it is much bigger and heavier than its predecessor, it is the most powerful and most expensive console of its generation, and it has struggled in sales against it's main rival the Xbox 360, it also compete against the Nintendo Wii arguably more indirectly as this is aimed more at the younger and social gaming market, the PS2 is more of a competitor to the Wii.
The PlayStation Portable (PSP) Edit
The PSP is the first handheld PlayStation. It was released in 2004.
The PS4 was released in 2013.