Frontside and backside are surfing, skateboarding, snowboarding and aggressive inline skating terms that are used to describe how a person approaches an obstacle or performs a certain trick. In Aggressive Skating, frontside and backside are types of grinds.
The names frontside and backside originate from surfing where they mean the direction the surfer is facing while surfing a wave. If the surfer is facing the wave, he or she is surfing frontside, otherwise he or she is surfing backside. The terms forehand and backhand are synonyms for frontside and backside but they are only used in surfing.
For performing rotational tricks on flat, like 180 kickflips, Frontside means that for the first 90-180 degrees of the rotation, you can see whats in front of you in the direction that your traveling. So a frontside 180 Ollie done in regular would be rotated towards the heels or anti-clockwise (clockwise for goofy skaters). Therefore a regular Backside 180 would be rotated to your toes or clockwise (anti-clockwise for goofy skaters), this is because you can not see what is in front of you for the first 90-180 degrees, only what was behind you as you are going into the trick blind. The only exception is when going fakie, a trick is considered frontside if it is done with the same motion that would be frontside if the person was facing forward to start.
For grinds, it's a lot easier, if you can see the rail/ledge in front of you as you approach it, this means you will be doing a Frontside grind, and if the rail/ledge is behind you, this means you will be doing a backside grind. This comes from when surfers would surf waves, if they could see the wave, they were surfing frontside, but if the wave was behind them they were surfing backside. The only exception to this is on banana bunny grinds, in which the terms are reversed.
In snowboarding, frontside and backside are referred to exactly like skateboarding when performing tricks. When turning in the snow, a backside turn in analogous to a toeside turn, and a frontside turn is analogous to a heelside turn.
In the air, backside means that you turn the front of your body into the rotation first and frontside means you turn your back into the rotation first. For example, a left foot forward (or regular) rider would rotate counterclockwise to throw a frontside 360 and clockwise to throw a backside 360.
In Aggressive Inline Skating, frontside and backside are used to describe the way the skater's feet are positioned when grinding on an object. For example, if a skater jumps on to grind a ledge with both feet landing on the h-blocks (the grindable area in the middle of the frame), with the toes facing the ledge, the skater is doing a frontside grind. Their whole body will be facing the object.
A backside is much the same, although when the skater jumps on, they will be landing with their back to the object, and with their heels facing the ledge.