Counter Strike was released in November 2000, and its 1.6 version was released in September 2003, Coutner Strike: Source in November 2004 is a tactical first person shooter game developed by valve corporation, both games takes the lead in online multiplayer gaming, LAN tournaments etc.
Now coming to the comparison-
Graphics Engine: CS 1.6 is built in GoldSrc Engine, CS:S is built in Source Engine, Source engine.
Graphics of Counter-Strike Source is more detailed than the former, lightning effects like bloom, HDR rendering etc. have been implemented in Source. It has detailed models, object physics calculation, detailed shadows, realtime reflections, realistic water etc.. Also it doesnt take a too-costly hardware to run the game, medium end PC will do fine. CS 1.6 graphics are simple, simple texturing, use of old pixel shaders, no advanced physics (like dropping of gun, rolling of barrels like that in source) and no bloom or hdr effect. Still its the most played game online and competes with CS: Source. Screenshot of Source Engine Graphics in de_inferno Screenshot of GoldSrc Engine in de_inferno
Shader model: Source shader models are very advanced and therefore object model details (like bomb model, gun models etc), texture details(texture resolution, multitexturing, transparency etc.) are all very high. CS 1.6 shader model is simple.
Recoil System: This is the in-game feature of both the games for the recoil of the guns, CS:S and CS 1.6 have different recoils of all the guns, that is, every gun has a different recoiling (how much it will recoil). But a person has to be at zero velocity to restrict maximum recoil in both the games, one cannot move while shooting with 100 percent accuracy. And a general view is, that source weapons has much less recoils than CS 1.6 weapons. Some other differences in weapons are that in CS 1.6 one can immediately open a scope in AWP (in 0 ms), letting him to fire the shot very fast, in source it takes some time (in some milliseconds) to open the scope, so there is a delay in the shot. In CS 1.6 its called quick zoom shot.
Bullet Peneteration: We mostly call these spams, CS 1.6 spams has become the part of every professional's gameplay, and to know the walls in map ehich are spammable. In CS 1.6 most walls are spammable, every box (green box and brown one) is spammable and from edge, every wall is spammable to large extent. CS:S have very less walls that are penetrable, but most wooden doors or boxes can be spammed. In CS:S, bullets through concrete walls only pass when bullet is hit at the edge of the walls. In short, peneteration depth of the walls is much less in Source than CS 1.6.
Smoke: In Source, the whole map may have smoke particles in the atmosphere(like dust) as if some wind is there, there is no smoke particles in air in CS 1.6, only smoke comes from a smoke grenade.
Source's smoke grenade takes much greater amount of GPU power than CS 1.6 smoke grenade. Both smokes are very different, in Source, smoke is more realistic.
Models: As already told, Source has detailed models than 1.6, however the model structure of most guns is the same except that of AWP. in source AWP is more longer and realistic and looks powerfull, 1.6 AWP is less detailed and person looking straight seems to look some what a bit down as if he is about to shoot the legs of enemy at middle range. Player models in 1.6 are not much detailed but animations are excellent. Though some player models may seem to have their upper body part too much forward like the Elite Crew, which doesnt look realistic.
Animations: CS 1.6 have a lot of bugs still(you may call it a bug or not), like switching between knife and gun, first of all, in first person view, knife seems to come from left then it immediately comes from right hand side or vice versa depending upon the where your gun model is). If you switch immediately after 1 stab of knife to a gun, it seems that player is taking out a gun in a wavy manner, that is, animation of knife still persists and merges with that of gun when one immediately switches after 1 stab of knife in air or somewhere else. The animation should be stopped just when switching, like when the player is planting the bomb, and then stops it and takes out a gun, bomb animation doesnt end. Then there is a matrix jump, when the player seems to float in air then come back to ground just like trinity did in the matrix. In CS 1.6, when player taps the crouch button (not press it, but tap and immediately release it), the player seems to levitate off the ground, and then when he again crouches while in air (that is two taps fast), player immediately crouches (pressing w and doing double tapping of crouch button is called duckwalk in cs 1.6) no crouching animation is there in 1.6. Source animation is fine, there are not much problems with it. There is no duckwalk in source, no matrix jump, nothing, and there is a crouch animation which 1.6 lacks.
Maps: Maps are same in both the games, but structure differences are many like, Ashort in dust2 of source is very wide area, doesnt have some boxes etc. Like that, overview of maps are the same. There are trees in Source, CS 1.6 dont have trees.
Bunnyhopping: Both the games include bunnyhoping, in source, bunny hop is somewhat slower than CS.
Silencer bug: In CS 1.6, a silencer can be put on m4 or USP even when switching guns in the middle of the animation of putting the silencer, in Source, if you switch the gun, there is no silencer put back on.
Flashbangs: They are more powerful in Source, player may confuse about the positions of enemies, player can be affected even when seeing the reflection of the flash or the light of the flash. CS 1.6 contains many flash bugs, like flashing between some walls make some other part of the map get flashed completely. (Which are not allowed now in some tournaments).
Boosting and jumping In CS 1.6 you can jump to top of the other teammate and take advantage of unatainable positions, while in source you cant do that, players shakes when they are on top of some other unflat object model. While there is a bug in source called as boost jump bug or trick. In which a player just shoots off the other player below him.