Difference between Half-life versions?

Half-Life: Source is Half-Life with Source. Half-Life 2: Lost Coast comes with Half-Life 2. Half-Life 2: Deathmatch is a separate title from HL1 and HL2.

Get Half Life 2,HL 2 Episode One,HL 2 Episode Two and the Black Mesa mod.

Different stories, it tells you in the game descriptions on the steam store.
Half-life, Half-life 2, Half-life 2 Episode 1 + 2 are the standard games, the rest are extra stories, or multiplayer games.

you know... sequels and stuff ?

Half Life Source is a slightly better looking version of Half Life, Blue Shift and Opposing Force are expansions of Half Life, the HL2 episodes are sequels to HL2, and the rest you shouldnt bother unless you're really into multiplayer.

half life is the first original game,
half life source was an updated engine of source engine,
half life blue shift is the story of a security card of black mesa,
half life opposing force same as blue shift ,
half life deathmatch source is the multiplayer mode of half life source{with the updated source engine},
half life 2 the sequel and all after this sequels,
BUT half life 2 lost coast was a demo to show to people the new source engine and half life 2 deathmatch is the multiplayer mode of half life 2

• Half-Life is the original
• Half-Life:Source is the remake in the Source (HL2) engine
• Blue Shift & Opposing Force are expansions to Half-Life
• Half-Life: Deathmatch Source is a Source remake of the Half-Life Multiplayer component.
• Half-Life 2 is the original Half-Life 2 release.
• HL2: Episode One & Two are the episodic follow-ups.
• Lost Coast is a tech demo showcasing engine improvements to Source.
• HL2 Deathmatch is the DM component of HL2 made standalone

Basically, you need to buy the Valve Complete Pack when the Summer Sale starts.

Maybe then buy Orange Box and Half-Life Anthology? It will got all of your list.

Half Life - the Original
Half Life Source - the same game remade in the source engine so slightly better graphics
Half Life blue shift - spin off from the first game you play a side character barry (made by gearbox)
Half Life Opposing force - sequel to blue shift
Half Life Deathmatch Source - came from a mod basically just a deathmatch mode for half life 1 source
Half Life 2 - sequel following half life 1
Half life 2 episode 1 - follows after hl2
Half life 2 episode 2 - follows after hl2 ep 1
Half life 2 lost coast - a short game to show off the new HDR feature sin the source engine
Half Life 2 deathmatch - same as half life 1 deathmatch but for the 2nd game, its free if you own hl2

Thanks guys, it was confusing me. I'll do as wuut said and buy only the main games :3

Half-Life - original
Half-Life: Source - original updated with new engine
Half-Life: Blue Shift - expansion, nearly same story from the point of view of a different character
Half-Life: Opposing Force - same as above
Half-Life Deathmatch: Source - a multiplayer game
Half-Life 2 - The sequel to Half-life
Half-Life 2: Episode One - Sequel
Half-Life 2: Episode Two - Sequel
Half-Life 2: Lost Coast - A short tech demo
Half-Life 2: Deathmatch - A multiplayer game

Just get Half-Life 2 and the 2 episodes. If you don't mind old graphics, get Half-Life. If you want a modernized fan-made remake, get Black Mesa (currently incomplete).

half life 2 deathmatch is an online game where you kill each other, half life and half life 2 is the main stories

Half-Life - first HL game
Half-Life: Source - same as above but with new the Source engine
Half-Life: Blue Shift - non-canon expansion, from the point of view of security guard officer (called Barney Calhoun) [made by Gearbox]
Half-Life: Opposing Force - non-canon expansion, you play as soldier and your mission is to kill the aliens.....and Gordon Freeman [made by Gearbox]
Half-Life Deathmatch: Source - same as HL1 multiplayer mode just with new the Source engine
Half-Life 2 The sequel to Half-life
Half-Life 2: Episode One - Sequel to HL2
Half-Life 2: Episode Two - Sequel to HL2:ep1
Half-Life 2: Lost Coast - A techdemo
Half-Life 2: Deathmatch - A HL2 multiplayer part (you could just buy this alone to play MP, but comes with HL2)

don't be lazy! you can always use google.

You forgot Half-Life: Uplink :D
(it's okay, everyone does)

Half Life and Half Life: Source are basically the same game, except one of them runs in Source and the other runs in GoldSCR.
Everything else has its own story line, except for Half Life Deatchmatch and Half Life 2: Deathmatch. Those two are just some multiplayer games without any story at all.

Well, you kinda got your answer. However, HL Source has a stupid AI compared to HL1 and Lost Coast was first intent as a HL2 chapter after or before Highway 17 (not sure which one) and then abandoned an released as a tech demo to demonstrate HDR.
And you missed Black Mesa - fan made remake of HL1. However, Black Mesa is not yet completed (you can read it in the description)

and then there are people who are complaining there are no half-life sequels...

Get all of them! They're the best single player FPS franchise ever made. Personally, I like the engine more for HL1 than 2, it just feels so... right. Then again, that's probably 3,000+ hours in TFC, 1,000+ in CS, and a couple hundred in DoD talking. XD