If you're an optimist, they are alternative ways of approaching representative democracy. If you're a cynic, they are excuses you make for not having representative democracy even though you're supposed to.
virtual representation was a doctrine proposed by the British government, spearheaded by Prime Minister George Grenville in the mid-1700s. It said that despite the fact that the colonies (particularly the colonies we now know as the United States) were in fact "represented" by Parliament, because even though they could not actually vote for members of Parliament and had no actual formal authority to make decisions in the British government, nevertheless Parliament was wise and responsible enough to take the interests of all British citizens into account worldwide.
As you may recall, this line of argument did not convince the US colonists, and led to the American Revolution.
selective representation is when some groups of people are represented more in government than others. In the extreme case, selective representation can mean that whole groups of people are made ineligible to vote; in the US women could not vote before 1920, for example. In other cases, it simply means that some groups are represented disproportionately---most of Congress is comprised of rich White men, despite them being a small fraction of our overall population. It is also possible to use selective representation intentionally, such as requiring a certain number of seats in the legislature be held by members of a particular ethnic group, even if that is not how the vote would have turned out. This disenfranchises some voters, but (it is hoped) for a greater good. In this way, one form of selective representation is used to try to combat another.