After having published a paper, am I still free to use & distribute it as I wish? Do I lose copyright if a journal accepts my paper and publishes it? Can I send my paper to colleagues, or is this technically illegal if they do not have access to the journal via their institution (or haven't bought it themselves)? (whether people do this anyways is a different question).

In short, is there anything I cannot do or that I would be restricted in doing after having published my paper in an academic journal?
In most cases, journals ask for transfer of your copyright to the publisher. In these cases, you cannot technically distribute copies of the article any more without the publisher's consent.

The details of what you can and cannot do, however, differ between publishers. You will get a form upon acceptance of your paper that lists the legal details and that the publisher requires you to sign before they publish the article.