"Swell" is a group of waves that travel across the ocean, created & energized by wind blowing on the back of the waves. “A swell” suggests a group of waves with a distinguishable height, period and direction, more than likely coming from a unique source (such as a specific hurricane, storm, or even a local wind system).
Below is a snapshot of wave height across the North Atlantic (click here to go to the chart). The red boxes highlight a couple of swells traveling towards Europe:
Here is the corresponding swell analysis graph for a spot on the West coast of Portugal — Supertubos:
All different swells arriving at a spot, having travelled across the ocean, are laid underneath the surf height in the swell analysis graph
I have highlighted the 2 different swells identified on the North Atlantic chart arriving on Friday evening and Saturday night. As Supertubos is open/exposed to the direction of the arrows, you can see that the resulting surf size at the beach increases with the arrival of both swells. Multiple swells will combine and interact with one another, to produce the surf (or more simply the breaking waves) you see at the beach.
As swells arrives at the beach, shallow water forces waves to slow down and rise up above the surface, morphing as it goes through a process known as "wave shoaling".
Surf is swell that has arrived in shallow enough water to rise up above the surface, and break. You can imagine that the lefthand side of the below diagram represents swell, and the righthand side represents surf.
A single wave in a swell has two dimensions: height and length. Period and height provide us with both the X and Y measurements of the wave, respectively. Measuring length in seconds might seem strange, but measuring a wave’s length in meters would be extremely difficult in practice. So, to get an idea of the wavelength of a wave, you simply measure the time it takes for a buoy to get "bobbed" up and down, in seconds — the time it takes for 2 successive crests/troughs to pass a fixed point.
Swells; a deeper dive
That height and length we mentioned will give us an idea of the "volume" of water the wave is moving.
If you imagine a wave with a long wavelength but small size (e.g. 2ft @ 20 seconds), you might find that — sitting on a boat — the up/down movement feels minimal. But, as the volume of water is quite large (due to the high period of 20 seconds); when this swell arrives at shallow water, it morphs in a way to produce large surf.
Likewise, you might have a swell with a height of 5ft and a period of 6 seconds... This would be quite typical of "stormy", or "messy" seas. On a boat, you will feel a lot of up-and-down movement, maybe even to the point of throwing up! The swell size might be large; but due to the small period of the wave, the "volume" of water that will be forced up above the surface as it arrives at the beach won't be so significant, and therefore will not necessarily result in "large" surf.