1998 was a blistering hot year, caused primarily by an enormous El Nino that year. 2005 was hot, too, and while some claim it was hotter than '98, my view is that it was a statistical tie.
Does this mean that global warming stopped in '98? (as argued in places like this) The answer is no. To illustrate, let's consider some synthetic data I made up.
First, let's assume that the human contribution to globally averaged temperatures looks like this:
This is 0.2 deg C per decade, similar to measurements over the past few decades. But as we all know, other things also affect our climate. A particularly good example is El Nino. During an El Nino, the globe warms considerably compared to non-El Nino years. El Nino's occur every few years, so let's assume that El Nino's contribution to global temperature looks like this:
Now let's sum them.
As one can see, the El Nino signal in our little example (and in reality) totally dominates the human-induced signal.
So if one wants to know how much "global warming" has occurred since 1998, one has to subtract out the influence of El Nino. If one does that, then 2005 is much hotter than 1998, and global warming is alive and well.
The important lesson to learn here is one of time scales. Looking at the temperatures from 1998 to 2005 means you have about 8 years of data. This is comparable to the El Nino cycle time. If one looked at much longer times (e.g., a few hundred years), the effect of El Nino would be less important and more obvious. To illustrate that, here's the same two time series extended out 200 years.
As you can see, the upward 0.2 deg per decade signal is quite obvious in this plot.
The same is true if the time series is much shorter than the time scale of the variation. That's why we don't have to worry about ice age/interglacial variations in our analysis of the warming of this century. But by picking a time scale that is comparable, it maximizes the confounding effects on any trend calculation.
People who argue that global warming stopped in 1998 are 1) clever advocates who are willing to mislead to win the argument or 2) don't understand much about the climate system.