What are some differences between playing online vs playing OTB (over-the-board)? If you are used to playing online, like myself, would playing OTB have a big impact on your play?
Disclaimer: I don't belong to any sort of chess club, so OTB might mean something a little different for me and others like me.
One difference for me is the time constraints - OTB games for me are quite casual, and I don't own a chess clock, so while the players might rib each other during a particularly long "think", there aren't any hard and fast time rules. On the other hand, internet games always have the clock running, for obvious reasons. My play is worse online, probably because of that. For one thing, I never have the time to set aside an hour for an internet game, so I tend to choose the 10 min ones, resulting in shallower play. My OTB games, on the other hand, tend to go a lot deeper.
I haven't noticed any objective effect on my OTB games from playing on the internet, though. Since going online, I've been able to appreciate the lack of pressure in casual OTB games. The net effect is that I feel I play better OTB, so I'm more confident than I used to be.
I grew up playing Chess OTB with my father, but have since switched mostly to online chess. I sometimes attend a local chess club, and there we play more OTB games. When I play, I find that I'm a little bit less comfortable with OTB play, but not because of time constraints. For me, it's simply that I'm less used to the look and feel of the pieces compared to online. Even the slanted view of the board, and the fact that the pieces are 3D is sometimes strange after a long break from playing OTB.
On the other hand, once I get used to the fact that I'm physically moving pieces and hitting a clock after each move, there's no real difference. The biggest difference at that point comes in how the time controls are set. Online, you can typically pre-move pieces (move during your opponent's turn, so as to use less time), but such an option doesn't exist OTB. The best you can do is have a "delay" setting on your clock.
You can expect many of your online opponents to use programs especially at slower time controls.
With OTB, there is none of that. You sit down, you punch the clock, and play. You talk to people about the game, get help from stronger players, play speed games between tournament rounds, and generally have a lot of fun.
I just wanted to make a comment regarding online ratings v. OTB ratings.
Most coaches that I have spoken to regarding OTB, USCF ratings vs an online long-time-control Internet rating suggest that the rating difference is generally between 100 and 130 points higher on the serious chess servers like ICC. This is generally said to be because either the pool of players is less skilled or the games are simply more informal.
I can personally move between OTB and computer-based play with no issues except for hitting the clock and writing down moves. When the game gets intense I frequently forget to write my moves and that is a very bad habit. But after 2 games or so I'm back in my stride and have all that worked out.
Hopefully come November when I get back to tournament play after years of not playing I will be able to just get back into the habit it without any issues.
I'm surprised no-one mentioned illegal moves or touch-move rules - which are pretty much the biggest difference between OTB (tournament) and online play.
Obviously the software won't let you make an illegal move, like moving a piece into the wrong square or ignoring a check. If you make an illegal move in a blitz game OTB you could forfeit the game altogether. Also, if touch-move is enforced and you touch a piece, this is the piece you have to move, if you can do so legally.
I find blitz harder to play otb than online but really the way one plays otb should be the same as how they would play online as it's the same game and the same person playing the game. The main differences I find are the 3d pieces and board/surroundings, which can be quite daunting especially if I don't get regular'ish' practice otb, and I also tend to hesitate a lot more than I do online. I believe the hesitation aspect comes from the heightened fear of losing otb as apposed to online due to the personalization factor (opponent and onlookers) and as a result I tend to take less risks nor trust my intuition and as a result play worse, spend too much time and calculate less. I think really it's more of a psychological issue rather than a visual one and requires more self belief and trust in ones intuitive skills.
Before talking about my experience, I'll just say that I grew up playing OTB first at a young age, then started playing online. From your question, I also assume you haven't played OTB at a tournament yet.
Online games with long time controls are quite different to OTB. Staring a long time at the screen, on the same position, strain the eyes, and concentration can drop with the temptation to open a new tab during opponent's turn, or start some music. OTB, at most, sitting for a very long time can be uncomfortable, and keeping a straight back is hard once the game has gone on for long. Nevertheless, it's easier to keep concentration OTB than online.
With rapid and blitz games though, the difference is smaller. One key thing in OTB though is the feeling of hitting the clock in zeitnot, which is much more stressing than moving a piece online. Same for casual games, but like it was pointed out, it's easier to set up variants (try 4 man Blitz, it's great :))
The biggest difference between both though is that you're more physically involved OTB. In terms of comfort, you can settle down however you want in front of your computer, but you can't bring more than some food OTB at tournaments. Moreover, you see and touch the piece physically OTB; moving the mouse has a duller sensation because it's been held for a while in the same position. In tournaments, there's also the striking silence rythmed by hit clocks, it's very impressive the first time you experience it.
For all these reasons, I believe there's a big difference between playing online and OTB. Online is more convenient. OTB has a stronger, more unique feeling to it. If you can, try experiencing an OTB tournament for yourself.
The biggest difference for me between OTB and online is that online, the game is recorded automatically, so if it turns out it was interesting, I can analyze it and keep it for posterity.
In OTB I don't generally write down the moves, so if it happens to be interesting I end up regretting not having a record of it.
Naturally, online is purely about the game, while OTB has more of a social aspect to it.
This also leads me to play a bit more aggressively, because reaching interesting positions gets a higher weight in relation to purely maximizing winning odds.
OTB it's easier to place handicaps to make games more interesting. I don't know of a Chess Server that allows rated handicap games. Lichess' simul offering is lackluster. OTB there's no problem doing time odds, simul, or even material odds (though I rarely find a player interested in the latter).