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.
In skateboarding, the definitions are confusing and are used very inconsistently depending on what kind of trick is performed. A trick that involves rotation (such as a 180 ollie) is called frontside if after the first 90 degrees of rotation the skateboarder is facing forward. The exceptions to this rule are fakie tricks, for which the definition is reversed. For example, a fakie frontside 180 (also known as a frontside half-Cab) looks like a nollie backside 180 but it is referred to as frontside because it is essentially a frontside 180 performed while going backwards. The fakie exception does not cover switch tricks, which are named like regular tricks.
For rotations a simple rule is: Regular Footed Skaters - Clockwise is Backside for Nollie, Fakie and Regular Stance but Frontside for Switch, Counter-Clockwise is Frontside for Nollie, Fakie and Regular Stance but Backside for Switch Goofy Footed Skaters - Clockwise is Frontside for Nollie, Fakie and Regular Stance but Backside for Switch, Counter-Clockwise is Backside for Nollie, Fakie and Regular Stance but Frontside for Mongo.
For slides and grinds, the trick is called frontside if the skateboarder approaches the obstacle with his chest facing it. This is somewhat counterintuitive in tricks like the backside board slide where the skateboarder approaches the obstacle with his or her back towards it but then turns 90 degrees and slides forward. The fakie exception mentioned above does not cover these tricks.
If you're in doubt what to name a slide/grind imagine doing the trick on a ramp. For instance the boardslide on a ramp is a backside move, though you'll have to ollie frontside to get into it on street obstacles.
Most often when describing a boardslide or lipslide trick or many other tricks, the reference to "frontside" is not used. Examples of tricks like this are: Lipslide and Backside Lipslide. Boardslide and Backside Boardslide. Noseslide and Backside Noseslide. Blunt and BackSide Blunt.. (It is intuitive to assume what a frontside is)...there are opposites to this like Rock and Frontside Rock & Invert and Frontside Invert. (Basically the perceived more difficult trick is the one that is clarified with either FS or BS)