This is a topic which is included in just about every guide to online poker, and it's not by accident. While there are certainly distractions that can arise in a live game, one of the benefits is that by physically sitting at a table with your opponents, your focus will tend to center on the game action pretty much. There's very little else to compete for your attention in fact.
In a real sense though, paying attention at a live game may be of a higher order priority than it is playing online, since there at least seems to be a lot more to observe. We don't want to make the mistake of assuming that there isn't much of value to be observed at an online game though. By letting our focus stray too much, we may often not act with the same degree of information as we would had we been paying attention more closely.
Playing poker in the comfort of your own home offers ultimate flexibility and convenience. We can play whenever we want under any conditions we want. However, this also brings to bear numerous distractions which may compete for our attention. Other people in the house, the TV, email, working on other projects simultaneously, listening to music, reading, thinking about other things, playing games, playing more than one table at a time, and so on all present the opportunity for our attention to be diverted. And when this happens, we at least potentially may be taken off our game.
This doesn't necessarily mean that you need to focus completely on a single table at all times. There will be distractions, some unavoidable, and some you actually will choose. The trick here is to keep these distractions to a manageable level, so that you're playing skill isn't compromised.
For instance, I often listen to music while I'm playing online. I don't find this distracting to any real degree. perhaps due partly to my becoming accustomed to it. This makes the time pass faster and makes my playing experience more enjoyable. In the event I really need to think a decision through more, I can always take off the headphones momentarily, although I don't really find the need to. Other players, perhaps those with limited experience, may find music too distracting at times though. So what's important with something like this isn't so much following a hard and fast rule about what to do and what to avoid while playing, it's more a matter of what effect a certain extraneous task may have on your game. This requires an honest assessment though, not just a rationalization :)
Sometimes things can arise which need to be done, and you may be in the middle of a game. This also doesn't necessarily mean that your game has to stop while this is going on, and it's going to depend on how well you feel you can accomplish both tasks at the same time. For instance, I do business on the internet in addition to my poker playing, and often situations arise which require my attention. If it's not too intensive - perhaps chatting with an associate or client, I don't really have a problem continuing my game while doing so. Other times I may have to step away from the table if something needs to be done that won't really allow me to devote enough attention to my playing at the same time. Or perhaps what needs to be done isn't too urgent and can wait until I'm done playing. Again, this is something you'll have to decide, and the key consideration here is whether you feel comfortable that you can multitask and maintain profitability at the poker table.
On the other hand, there are a lot of players who are comfortable playing multiple tables at the same time. I've tried this and I can honestly say that I'm not comfortable with it at all. Perhaps this is due to the fact I played so many years exclusively at live games, before online poker became invented, and I've been conditioned to do a lot more observation than many of today's online players. It's not that I can't play more than one table, it's more like I feel out of touch when I do, and never feel like I know what's going on properly at any of them. Other players enjoy multi tabling and are very comfortable in this environment. Again, this comes down to the player more than anything.
There are some general rules of thumb here nonetheless, which you should consider when gauging what sort of level of focus is required for you to succeed. Any interruptions should be brief and fairly infrequent, and not take you away from the table too long. Things like writing email, watching TV, or anything else which competes for your attention for any real length of time should be avoided. If you really need to do something of this nature, it's almost always better to step away from the table and resume when the tasks are competed, or postpone them to a later time when you're finished your playing session. You want to definitely avoid tasks which have you returning to the game only when it's your turn to play, since there's a lot that you'll be missing, in addition to suffering a loss of focus generally, where your attention span has to divert too much from the outside task and back to the poker table.
Of course, with Poker Edge, the software does a lot of the work for us in terms of monitoring the play of our opponents, but it still pays to be fairly attentive, especially when playing no-limit, where you can gain extra valuable information by closely observing betting patterns.
In the end, only you can really know what distracts you, and to what degree, while playing online poker. You are your own boss in this enterprise, and it's up to you to make your assessments in this area as honest as possible, in order to ensure you do what's right for you.