This page is just a reference of all the known Steam APIs, I do not know how they work. Please do not email me with questions.
GETGetGlobalAchievementPercentagesForAppv2
https://api.steampowered.com/ISteamUserStats/GetGlobalAchievementPercentagesForApp/v2/
NameValueTypeRequiredDescription
uint64 Yes GameID to retrieve the achievement percentages for
GETGetGlobalStatsForGame
https://api.steampowered.com/ISteamUserStats/GetGlobalStatsForGame/v1/
NameValueTypeRequiredDescription
uint32 Yes AppID that we're getting global stats for
uint32 Yes Number of stats get data for
string[] Yes Names of stat to get data for
uint32 NoStart date for daily totals (unix epoch timestamp)
uint32 NoEnd date for daily totals (unix epoch timestamp)
GETGetNumberOfCurrentPlayers
https://api.steampowered.com/ISteamUserStats/GetNumberOfCurrentPlayers/v1/
NameValueTypeRequiredDescription
uint32 Yes AppID that we're getting user count for
GETGetPlayerAchievements
https://api.steampowered.com/ISteamUserStats/GetPlayerAchievements/v1/
NameValueTypeRequiredDescription
click to setstring Yes access key
uint64 Yes SteamID of user
uint32 Yes AppID to get achievements for
string NoLanguage to return strings for
GETGetSchemaForGamev2
https://api.steampowered.com/ISteamUserStats/GetSchemaForGame/v2/
NameValueTypeRequiredDescription
click to setstring Yes access key
uint32 Yes appid of game
string Nolocalized language to return (english, french, etc.)
GETGetUserStatsForGamev2
https://api.steampowered.com/ISteamUserStats/GetUserStatsForGame/v2/
NameValueTypeRequiredDescription
click to setstring Yes access key
uint64 Yes SteamID of user
uint32 Yes appid of game
PUBLISHERPOSTSetUserStatsForGame
https://partner.steam-api.com/ISteamUserStats/SetUserStatsForGame/v1/
NameValueTypeRequiredDescription
click to setstring Yes access key
uint64 Yes SteamID of user
uint32 Yes appid of game
uint32 Yes Number of stats and achievements to set a value for (name/value param pairs)
string[] Yes Name of stat or achievement to set
uint32[] Yes Value to set