On sites such as this one, people are often concerned about non-lawyers "giving legal advice" or "practicing law without a license". What, exactly, constitutes such actions? I'm particularly interested in the situation in the United States, since that's where the StackExchange servers are.
It depends highly on which jurisdiction you're referring to. This is one of the areas where it varies state to state. "Practicing law" is not generally defined in statute, but the Wikipedia definition defines it as (emphasis mine):
giving legal advice to clients, drafting legal documents for clients, and representing clients in legal negotiations and court proceedings such as lawsuits, and is applied to the professional services of a lawyer or attorney at law, barrister, solicitor, or civil law notary.
Wikipedia defines Legal advice as
the giving of a professional or formal opinion regarding the substance or procedure of the law in relation to a particular factual situation.
and displays the following enforcement provision (emphasis mine):
Criminal laws and enforcement of "Unauthorized Practice of Law (UPL)" statutes is the organized bar's preferred method. Thus, New Jersey has a law which makes it a “disorderly persons offense” to knowingly to engage in the unauthorized practice of law, and a “crime in the fourth degree” to commit UPL if one (a) creates a false impression that one is a lawyer; (b) derives a benefit from UPL, or (c) causes an injury by UPL.
See also IANAL, which has some information about usage on the Internet:
The case law standard for determining what comments cross the line is generally "the application of law to facts specific to an individual seeking legal advice".