ReplicatedStorage
This is a general storage the server automatically syncs to all clients. The replication is one-way, so if a client places anything in here, it will not replicate to the server or any other client.
Normally, scripts that are placed in ReplicatedStorage will not automatically execute.
ServerStorage
This is a general storage for things that remain only on the server. Although clients have a service named ‘ServerStorage,’ this is for consistency only and anything placed in here will not be visible or accessible to clients.
Normally, scripts that are placed in ServerStorage will not automatically execute.
ServerScriptService
Unless disabled, Scripts that are placed in the ServerScriptService are automatically executed by the server.
This is server-side only so like ServerStorage, objects placed here are not replicated to clients.
StarterGui and StarterPack
When the player character spawns or respawns the PlayerGui and player Backpack are emptied out. Contents in the ‘StarterGui’ and ‘StarterPack’ folders are then copied to the PlayerGui and Backpack respectively.
As the name implies, the StarterGui is generally used for GUI elements, though this is not limited to only GUI elements.
StarterPlayer
The StarterPlayer service modifies the default settings and behavior for players.
Custom characters can be placed in here to override player configured characters.
When a character spawns or respawns, scripts in the StarterCharacterScripts sub-folder are copied over into the character.
On the other hand, LocalScripts in the StarterPlayerScripts sub-folder are copied into the PlayerScripts container only once when the player enters the game.