Much of surf forecasting is figuring out what happens to swell as it enters shallow water near the coast. Of course the most dramatic effect is the one we're most interested in as surfers, the final breaking phase.
As waves enter shallow water they slow down and they change shape, increasing in height. Longer period swells will increase more during this process than shorter period swells, which are steeper to start with. Once they reach a water depth of approximately 1.3 times their height they start to break. How rapidly this happens is affected by the local sea bed. On a gently sloping beach with light winds the wave will gradually increase in height eventually the top starts to spill gently forward. These waves break slowly, they're definitely surfable and can form the staple part of many surfers diets but are often slower to ride.
Alternatively the wave encounters shallow water more abruptly, a rock reef or a steep beach with well defined sand bars, in this situation the wave rapidly changes shape and the lip plunges forward in the most extreme case producing a hollow barrelling wave. This sort of wave is the stuff of magazine covers, surf videos and a challenge for confident and competent surfers.
Finally it's worth noting if the beach is steep enough the wave simply surges up the beach, these kind of waves are not surfable but explain why not every beach that receives swell has good surfing potential.
The tide can have a dramatic influence on the breaking wave – not by the effect of it's current on the swell (although there is an effect) but simply by changing the depth of the water. A common structure for sandy beaches in relatively shallow areas of coastal water would be to start with a gentle slope and finish with a steeper section towards the shoreline. At a high tide these beaches can offer enough water depth that waves simply surge up the steepest section and can't be surfed, but when the tide drops a long, slow spilling wave may occur. With only subtle variation in steepness a similar beach might offer the same slow wave at lower tides and might become a faster barrelling wave towards high tide and variations are as many as the changing contours of the sea floor.
A small rock reef can be considerably more dependent on the tide, not creating a breaking wave at all when covered by a high tide and then moving from slow spilling wave to heavy barrel and ultimately ending up as dry rock when fully exposed by a low tide.
Another element in this process is that sandy beaches are constantly changing shape. As waves move over the sea bed they interact with the sand and move it, generally towards the shore. Likewise as waves break there is a movement of water towards the shore, this water needs to return and this flow also has an effect generally moving sand away from the beach. Together these processes change the overall shape of the area in which waves are breaking and create ridges and troughs. At it's best regular swell can carve a long steep bank of sand that creates a long breaking wave that's steep but breaks for some distance, perfect beach break surf. At worst one long flat bank from one end of the beach to the other creates a giant closeout wave that breaks along it's entire length in one go and offers no kind of ride.
It's our experience that 'the banks' does become something of a catch all explanation for everything that changes from day to day at the local beach. Effects explained by swell period, direction, wind and other factors become something to do with the state of 'the banks' in the minds of many surfers. None the less the state of a sandy beach does change on both a regular basis and as part of longer cycles and it can have a very pronounced effect on the surf. You can't change them of course, but you can pick your beach and your state of the tide based on your recent experience.