• installation
• troubleshooting
• YSI_COMPATIBILTY_MODE

PAWN scripting improvements (i.e. new language features).

• y_functional
• y_hooks
• y_inline
• y_malloc
• y_remote
• y_stringhash
• y_timers
• y_unique
• y_va

Core libraries, used almost everywhere else.

• y_als
• y_cell
• y_debug
• y_master
• y_profiling
• y_testing
• y_utils

Data structures and algorithms.

• y_bintree
• y_bit
• y_iterate (AKA y_foreach)
• y_hashmap
• y_jaggedarray
• y_playerarray
• y_playerset
• y_sortedarray
• y_sparsearray

Optional features.

• y_extra
• y_files
• y_streamer_plugin

Libraries that provide information about the game.

• y_vehicledata

Libraries for managing players.

• y_groups
• y_languages
• y_text
• y_users

Libraries for controlling the server.

• y_colours (AKA y_colors)
• y_flooding
• y_lock
• y_punycode
• y_scriptinit
• y_stringise (AKA y_stringize)
• y_td

Libraries for interacting with persistent data.

• y_amx
• y_bitmap
• y_ini
• y_php
• y_svar
• y_uvar
• y_xml

Libraries that have in-game visible effects.

• y_areas
• y_classes
• y_commands
• y_dialog
• y_loader
• y_properties
• y_races
• y_zonenames
• y_zonepulse

No-one actually knows! The original idea was "Y_Less' Server Includes", but "Script" and "Server" were frequently intermixed, and there are more devs than just Y_Less now, so the "Y" just became a recursive acronym for "YSI". There are now several official meanings, each incorporating different aspects of YSI:

Core libraries.

Coding libraries (pawn language extensions).

Game related stuff (commands, properties, text etc).

Extras, like login and commands.

The macros (though in their defense, writing macros to do the parsing within the compiler limitations is VERY hard).

y_tho.