Pressure and spike are the Yin and the Yang of the best tactical PvP games. They are two opposing tactics in gameplay, working with and against each other. Knowing when to use one or the other and when to switch is the hallmark of a good player, and having such decisions as a natural part of a game is the hallmark of a clever PvP design.
What do these concepts mean?
Pressure: Literally, putting pressure on the opposing team. You probably won't win/score directly by applying pressure, but you will decrease their margin for error. This sets you up to be able to...
Spike: A co-ordinated effort to win/score.
I'll use Left 4 Dead as a nice example. The Infected team must continually harass the Survivors (pressure), and eventually must co-ordinate an assault to really lay the hurt on the Survivors and hopefully incapacitate someone (spike).
Now consider this - the higher the life totals of the Survivors, the less likely that a given spike will be successful. Hence the need to pressure the Survivors down - you need to take action that may not immediately secure your victory, but will instead enable you to achieve a spike in the future that will.
For example, imagine you are faced with attacking a target with low health or a target with high health, who are both separated a little from the group. This will happen roughly a bajillion times per match. If you attack the high health target, you reduce his health lower which increases the chances of a future spike incapacitating him and hence achieving the ever-juicy team wipe. If you attack the low health target, you may incapacitate him right now (taking him out of the equation), but if your team-mates aren't around and ready to take advantage of the situation, he will just be helped back up and healed. Plus, he was probably going to get healed soon anyway (his health was low).
So, you need to pressure to be able to spike, and you need to spike to avoid all your pressure being invalidated. The Yin, and the Yang.
Another example of a game that revolves around this concept is Guild Wars. The preferred build of the top guilds is the so-called "balanced" build. Warriors will chase squishy targets around and hit them to build their spiking skills, Mesmers will attempt to lock out key skills to facilitate damage, Monks will protect targets that look like they may be subject to a future spike, Rangers will spread debilitating conditions and DoT effects, etc. Soon, everyone's health and energy starts to get a little low (pressure). The lower a player's health, the less the margin of error that the Monks (healer characters) have in order to save them when the inevitable spike occurs.
The beauty of Guild Wars is that the more you are pressuring the other team, the more disruptive you are to them, and so the less their ability to spike you. In this way the best defense can be a good offense. But, if you pressure them too aggressively, you may find yourself overextending or not having enough energy to overcome the enemy's next spike. It's always a trade-off, this Yin and Yang of competitive PvP.
This idea applies to sporting games and I suspect pretty much any endeavour that pits two or more human competitors against each other.