When renewable energy advocates talk about phasing out coal-fired power plants in favor of renewables, they'll often use one of a pair of phrases to describe a power plant's output: "base load" and "peaking," a.k.a. "peaker." Some plants, like coal-fired and nuclear power plants, put out base load power. Others, like solar and most gas-fired power plants, generate "peaking" power. You'll sometimes hear statements to the effect that solar panels won't replace coal because coal is base load and solar is peaking.
It almost sounds as though the two are different flavors of electrical power. They're not. Base load and peaker power plants feed the same electrical power into the grid. The difference between base load and peaking power isn't in the power itself: it's in the economics and engineering limitations of the power plant.
Electrical power demand rises and falls during the course of a typical day. We tend to use less power at 2:00 am than we do at 2:00 pm. Even in our 24/7 world, most of us wake up in the morning, turn on our appliances, crank up the air conditioning in mid-afternoon if we're hot, turn on electric lights in the evening for a few hours, then turn them off again and go to sleep. At that point our power demand slows again. We need power for streetlights and traffic lights, hospitals, police and fire stations and businesses with graveyard shifts, a trickle to charge our phones and laptops. Chart out power consumption over the course of the day and you'll see a peak in the afternoon and evening:
As you can see, even when California is on "standby" setting, around 5:00 am or so, we still consume a considerable amount of power. That's the power consumption base load. And then when we start using air conditioning, and to a slightly lesser extent when we start using electricity for lighting in the evening, our power consumption peaks.
Which is a good first-approximation definition of "base load" power -- the minimum amount the grid has to have to run society -- and "peaking" power, which provides a cushion against peaks both anticipated and unanticipated. Like most things electric power related, things are more complicated than that if you look closely. Base load power doesn't stay constant during the day: it increases as demand increases, and so base load actually has peaks and valleys. It gets confusing.
But most of us don't need to examine the confusing details. Base load power is the day-to-day steadfast power we need 24/7, and peaking power is what we fire up when we need more.
Which is where the differences in sources of that power become relevant.
A power plant supplying base load power needs to be able to run for months on end without needing to be taken down for maintenance, and it's best if the fuel costs are relatively low. However, since base load power plants are rarely taken offline, it's not a huge problem if it takes them a while to start up.
A peaking power plant is one we can switch on when we need additional power, which will come online without much delay and start generating power on a moments' notice. As peaker plants are used for less time over the course of a year, it's not as crucial that the cost of fuel be low.
Typical base load power plants are coal-fired, nuclear and hydroelectric. Geothermal can also provide base load power. Base load power plants tend to be expensive to build, and coal and nuclear take days to reach full power once fired up. But fuel costs per kilowatt generated tend to be low, at least if you don't count the ecological costs.
Peaking power plants have traditionally been fueled by either natural gas, diesel oil, or jet fuel. The last two are significantly more expensive than gas, especially since the advent of fracking has pushed natural gas prices through the floor. Most peak power in the US comes from gas-fired plants. Despite gas' low price these days, peak power remains more expensive per kilowatt than base load. Hydro can also be used as a peak power source, as ramping up power production from a hydroelectric dam is generally a matter of letting a bit more water in through the turbines.
Solar power plants, by virtue of using the sun as fuel directly, can only produce power when the sun is shining (or, if expensive storage is added to a solar thermal plant, for a few hours afterward). As sunny afternoon hours more or less coincide with peak electrical demand, solar power plants are peaker plants, and will be until engineers make either thermal or grid storage a reality.
There's also an intermediate kind of power plant, referred to as "load-following" plants, in areas with high electrical demand. Load-following plants supplement the power produced by base load plants, but run for longer periods of time during a typical day -- or 24 hours, but with lower output at night. In the US, load-following plants are generally gas-fired or hydro, though nuclear-heavy Chicago does use nukes as load-following plants as well.
Wind, by the way, is an odd person out. In most places it's not constant enough to be base load, and not reliable enough to provide a secure source of peaking power. If we get to the point where large numbers of wind turbines in widely separated locations are all hooked together through the grid, some of that unpredictability will go away: wind dying down in one place might well be made up for when it picks up somewhere else. For now, wind tends to be a continually moving monkey wrench in the engineers' careful daily planning of power supply and demand.