Fishing can be performed at any body of water - bridges, lakes, etc. This is a reaction-based minigame, requiring you to type reel after a random amount of time as quickly as you can so the reward doesn't get away. There's many things you can get from this - trash, food, crates, materials, and levelling up increases the range of rewards you can get.
The higher your fishing level, the higher the rank of material you may retrieve while fishing. There is no set list of materials you can retrieve by fishing, every single material in the game is eligible, though as there are numerous materials this isn't the best way to search for a specific material you want.
Alchemy is done with materials using the !compound or !brew commands. By combining different materials, you can create better materials, potions or even weapons and armour! Try it out for yourself, there's so many recipes that I can't even list them all here.
Scavenging can be done on almost any tile in the world, and lets you gather materials from the area. The amount you get is random, determined by your scavenging level. Use !scavenge anywhere where it's an available action. If a tile is depleted, it'll be refreshed at 12am midnight (GMT) every day.
Salvaging is the act of deconstructing equipment you have into a base reagent material. This is based on rarity, not the item itself. Salvaging a Common item will always give you Recycled Parts, Magic will give Arcane Reagents, Rare will give Crystallised Energy and Unique items will give you Forgotten Souls. Raising your salvage level will increase how many materials you get back, as these are used in crafting high-end equipment, this is a fairly important life skill.
Woodcutting is the act of chopping down trees for logs. Different forest types have different logs, and the amount you get depends on your level.
Mining allows you to dig for gems, treasure and artifacts in quarries. Your mining level determines the amount of rewards/actions per mine as well as the chances of finding obstacles.
Increases damage you inflict on others, and slightly raises your chance to hit.
Decreases damage others inflict on you.
Gives you a chance to critical hit and to evade attacks, while also slightly increasing your chance to hit.
Raises damage of spells and effectiveness of some magic abilities.