Vanilla Content IDs
Terraria uses numbers to reference various pieces of content in the game. Each of these numbers usually has a corresponding ID name. Modders can use these ID names to easily reference vanilla content. For example, a modder can type Player.QuickSpawnItem(source, 22); to spawn an Iron Bar item at the players location. 22 is confusing, however, so the modder can type Player.QuickSpawnItem(source, ItemID.IronBar); instead. There exist ID classes for many different types of content, such as Items, Projectiles, Tiles, and so on. Using any ID class requires using Terraria.ID; at the top of the file.
Do note that the values of these IDs overlap. Both ItemID.IronBar and TileID.Demonite have a value of 22. It is important to make sure you are using the correct ID class for the type of content you are attempting to use. Mixing these up will result in bugs. For example, Player.QuickSpawnItem(source, TileID.Demonite); would spawn the Iron Bar item instead of the Demonite item.
This page lists info on using different types of IDs and links to the official Terraria wiki for the actual values. On these lists, there is typically an Internal name column corresponding to the actual ID name that a modder can type to get the numerical value. These lists can be useful for a quick reference, but don't waste your time looking up each ID while making your mod. We expect modders to be using the autocomplete feature of their IDE to automatically type out these IDs. Typically the IDs mirror the English name of the items, so autocomplete should be sufficient for modding. On rare occasions you might not find the ID you are looking for, if that is the case you can use the wiki links to discover the actual ID name. Also note that if Terraria recently updated and tModLoader has yet to update, the newly added ID values corresponding to content added in the most recent updates will not exist in tModLoader.
The Data IDs page on the official Terraria wiki has links to many listings of content IDs. The rest of this page will go into more depth about content IDs commonly used in tModLoader modding..
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Most ID classes have a Count field corresponding to end of the listing of IDs. You can iterate from 0 to SomethingID.Count in a for loop if you need to run some code for each piece of content.
Many ID classes have a Search field that has methods that can be used to retrieve ID names from the corresponding number value, and vice versa. This can be useful for making debugging tools. Modded internal names can also be retrieved in the same manner, but remember that modded IDs can change between game reloads. If you are trying to track content between game sessions, use classes like ItemDefinition.
Contains in many ID classes is an inner class called Sets, these classes store data corresponding to the content. For example the TileID.Sets.HasOutlines set tracks which tiles have outlines, hinting to the game to load and draw the outlines when the tile is hovered. Modders typically assign to these Sets in SetStaticDefaults methods. For example, a modder would write TileID.Sets.HasOutlines[Type] = true; in the SetStaticDefaults method of their ModTile class to tell the game that this tile has outlines. See ExampleMod for various usages of other Sets.
Item IDs page on the official Terraria wiki
Some common items with not obvious names: Luminite Bar is actually LunarBar, Luminite Bullet is actually MoonlordBullet, and Sky Dragon's Fury is actually MonkStaffT3.
As a reminder you use these values either by writing ItemID.Name or just Number, do NOT write ItemID.Number:
recipe.AddIngredient(ItemID.Gel); // Good
recipe.AddIngredient(23); // Works, but is hard to read
recipe.AddIngredient(ItemID.23); // WrongProjectile IDs page on the official Terraria wiki
Projectile IDs are typically used in Projectile.NewProjectile methods to spawn projectiles into the world. Many projectiles have multiple versions with Hostile or Friendly attached to the name of some of the duplicates. These indicate that the projectile is either friendly or hostile, use the one that matches the damage behavior you intend. If there isn't a version of the projectile you need, you may have to make your own version as a ModProjectile.
ProjAIStyleID.cs on the tModLoader GitHub
Projectile AI styles (use for Projectile.aiStyle) do not have official names. Included in tModLoader is the Terraria.ID.ProjAIStyleID class which gives names to each.
NPC IDs page on the official Terraria wiki
Negative IDs represent NPCs that share a type with an NPC with a positive NPC. If you need to differentiate between these variants, you'll need to check netID to determine which variant is being checked.
NPCAIStyleID.cs on the tModLoader GitHub
AI page on the official Terraria wiki
NPC AI styles (use for NPC.aiStyle) do not have official names. Included in tModLoader is the Terraria.ID.NPCAIStyleID class which which gives names to each. The official wiki has another listing with more detailed descriptions. Use both pages to find the information desired.
Tile IDs page on the official Terraria wiki
Tile IDs represent the type of tile. Tile IDs correspond to the Tile.TileType at a coordinate. Many furniture tiles listed have a number in the Sub ID column, this corresponds with the style concept explained in the Basic Tile Guide. For example, most chests in the game are listed as the Tile ID of 21, but have different Sub ID/style values to differentiate them.
The wiki does not list the internal names, those can be found on the Vanilla Tile IDs page.
Wall IDs page on the official Terraria wiki
Wall IDs are used in Tile.WallType. Many walls have 2 variants, and unsafe and a safe variant. Safe variants are player placed walls that don't have the same enemy spawning properties of the unsafe variants.
Buff IDs page on the official Terraria wiki
Use these in methods like Player.AddBuff and NPC.AddBuff. Note that many buffs only affect players or enemies, but not both. If Terraria doesn't have a way to give a buff to an enemy, it likely isn't programmed to work with NPCs. The same is true for buffs that aren't ever applied to players.
Due to their nature, dust are hard to identify, use the Finding Vanilla Dust guide to find Dust IDs.
Sound IDs page on the official Terraria wiki
Sound IDs can be used to play specific existing sounds in a mod. The Adapting Vanilla Code section of the Basic Sounds Guide goes over how to use the Sound ID information on the wiki to find and play specific Terraria sounds.