There are five core skills in Stardew Valley — Farming, Fishing, Foraging, Mining, and Combat. Leveling these skills up provides you with proficiency in your tools (making them use less energy), new crafting recipes, and more.
But the most beneficial rewards for leveling up your skills are the professions. These are unlocked at levels 5 and 10.
At level 5, you'll be given a choice between two options. These options both lead to another two options at level 10, as a sort of miniature branched skill tree. Here's what each of these professions does.
Farming
Farming is the skill associated with crops and animals. Every level you get in Farming increases your proficiency with a hoe and watering can, reducing the amount of energy it costs to use them. At higher farming levels, you also have a higher chance for your crops to be of better quality.
Farming experience is earned by harvesting crops, petting your animals, milking cows and goats, picking up animal products (except truffles — more on that later), and shearing sheep.
Farming Level 5: You are given a choice between Rancher or Tiller. Rancher makes animal products sell for 20% more good. Tiller makes crops sell for 10% more gold.
Farming Level 10 if you chose Rancher: You now have a choice between Coopmaster and Shepherd. Coopmaster causes coop-dwelling animals to become friendly toward you more quickly, cuts incubation time in half and increases the quality of coop products. Shepherd causes barn-dwelling animals to become friendly more quickly, allows sheep to produce wool more frequently, and increases the quality of barn products.
Farming level 10 if you chose Tiller: You're now given a choice between Artisan and Agriculturist. As an Artisan, your artisan goods will sell for 40% more gold. With the Agriculturist profession, all of your crops will grow 10% faster.
Mining
This may come as no surprise, but mining will improve your Mining skill. Anytime you destroy a rock, whether it be with bombs, your pickaxe, or by a monster, you'll gain Mining experience. Every Mining skill level will give you more pickaxe proficiency.
Mining Level 5: You'll be given a choice between Miner and Geologist. For Miner, you'll find an additional ore in every vein. Choose Geologist, and there'll be a 50% chance to gems in pairs when breaking a gem node.
Mining Level 10 if you chose Miner: If you are a Miner, you'll be given a choice at level 10 between Blacksmith and Prospector. Blacksmiths will receive an additional 50% bonus when selling metal bars. Prospectors have a doubled chance of finding coal.
Mining Level 10 if you chose Geologist: If you chose to be a Geologist, you'll have two choices at level 10 — Excavator and Gemologist. Become an Excavator and your chance of finding geodes is doubled. Opt to become a Gemologist and all gems you sell will net you 30% more gold.
Foraging
Foraging isn't just the skill related to foraging items from around the valley — it also applies to chopping down trees. For every Foraging level, you'll become more proficient with an axe.
You'll receive Foraging experience for doing the following: completely chopping down trees, removing stumps, large stumps, and logs, picking up forageables off the ground, harvesting spring onions, picking up truffles, collecting fruit from the cave (if you chose bats), and harvesting items from Wild Seeds.
Foraging Level 5: You'll have to choose between Forester and Gatherer. Become a Forester and you'll receive 25% more wood when chopping trees, stumps, and logs. If you choose to be a Gatherer, you'll gain a 20% chance of a double harvest when foraging items (this includes truffles).
Foraging Level 10 if you chose Forester: Now, you'll have to choose to become either a Lumberjack or a Tapper. Lumberjacks will be able to harvest hardwood from any type of tree. Tappers will gain an additional 25% gold when selling syrups.
Foraging Level 10 if you chose Gatherer: Choose to become a Gatherer at level 5, and you'll have to choose between Botanist and Tracker at level 10. If you're a Botanist, every item you forage will be its highest possible quality (including truffles). Become a Tracker, and there will be small arrows on screen that guide you toward forageables, artifact spots, and panning locations around the valley.
Fishing
Fishing isn't just the skill for catching things with a fishing rod — it is also leveled up by harvesting crab pots, catching trash, and reeling in seaweed and algae.
At every level, your fishing rod proficiency increases. However, there are other rewards for leveling up your Fishing skill, though. Each level also increases the size of your bobber bar, decreases the amount of time before fish bite, and increases the minimum size of fish you can catch.
Additionally, levels 1, 4, and 8 increase the distance you can cast by one tile each.
Fishing Level 5: Time to make a choice between becoming a Fisher and becoming a Trapper. Fishers will receive an additional 25% bonus when selling fish, whereas Trappers will need fewer resources to craft crab pots.
Fishing Level 10 if you chose Fisher: Choose to become a Fisher at level 5, and you'll get the choice between Angler and Pirate at level 10. Anglers sell fish for a 50% bonus. Pirates have twice the chance to find treasure when fishing.
Fishing Level 10 if you chose Trapper: If you became a Trapper at level 5, you'll have to opt between being a Mariner and a Luremaster at level 10. Mariners have the privilege of their crab pots no longer producing junk items. Become a Luremaster, though, and your crab pots won't require bait.
Combat
Killing monsters will increase your Combat skill. There is no tool proficiency associated with leveling up your Mining skill. Different monsters award different amounts of experience.
Combat Level 5: Your first choice in Combat professions is between Fighter and Scout. Fighters will deal 10% more damage with all of their attacks and have an additional 15 max HP. Those who choose Scout will have an increased (50%) critical strike chance.
Combat Level 10 if you chose Fighter: If you became a Fighter, next you'll have to choose between Brute and Defender. Become a Brute and you'll deal 15% more damage to enemies. Defenders receive an additional 25 max HP.
Combat Level 10 if you chose Scout: If you became a Scout, you'll choose between Acrobat and Desperado at level 10. Acrobats have a halved cooldown time on special moves. Desperados double the damage that their critical strikes do. The amount of damage a critical strike does is a complicated formula that accounts for your weapon, your rings, and more. Desperado doubles the damage after accounting for all bonuses.