"Metroidvania" is a term used to describe a sort of exploratory 2D platforming game. It comes from the original games of the Metroid series and some of the middle-era of Castlevania games (most notably "Symphony of the Night", which is what primarily gave birth to the term).
The most defining feature is usually the world. Rather than have set stages or a world map, the entire game takes place inside one giant map, which you must traverse about. Not just any map, but typically a very complex map filled with all kinds of secrets. The map is typically divided into sectors, more in an aesthetic fashion to denote that the denizens and dangers will be different. Sometimes you have teleports to assist you in travel, but other times you just have to remember where to go in the map.
There's various elements at play in the exploration. There's typically tidbits hidden in spots, or puzzles that have to be returned to when you find appropriate equipment. The exploration is also non-linear - your limits of progression through the map are by various tools or powerups that you need to acquire, but the order in which you must acquire some of these tools is not set. And in many choices, there are many paths to get those tools, or even get past obstacles without those tools.
A very popular element of these games, especially in the Metroid franchise, is the concept of "sequence breaking". Using special tricks or otherwise mastering the utility of your basic abilities, you can sometimes access items and areas far earlier than the game expects you to.
As far as I know, Metroidvania games have always been 2D platforming games. Aside from the elements of exploration, the actual gameplay of Metroidvania games vary widely.
Best Answer
Interp is short for interpolation, a computer science term (technically math, but that's unrelated).
In Counter-Strike and CS Source the server interpolates based on your previous position your next position. Since the server determines 'where' you are the decision of this calculation determines where you can get hit from. Likewise your client also makes a similar calculation as to where you and other players are. The result is by tweaking the interpolation coefficients you can trick your client into showing the various players in different positions than the server believes them to be.
Since interpolation is a guess, and its effect changes your viewing experience, players often change it for optimal performance. It is possible to change it to cheat, but since interpolation errors occur regardless it is harder to detect. In reality anyone accusing another person of interp is just accusing them of cheating; often an inane accusation.
A longer discussion of interpolation techniques can be found here, however I'll give you a simple example of linear interpolation:
yn=y0+ n * dy / dt
As the acceleration in the y direction may have changed between y0 and yn it is possible this equation produces inaccurate results. To prevent this often times low values of t are used, in CS this is typically .01, though some players set it to .009