Universe:

Indicates the top level Steam system for the account. See the Universes table.

Account Type:

Indicates exactly what it says, the type of the account. See the Account Types table.

Instance:

A big unknown, but is usually "1" for accounts with "Individual" Account Type and "0" for Groups.

Account ID:

32-bit unique identifier for the Steam account. Sometimes incorrectly referred to as the "32-bit Steam ID".

Legacy Text:

Exists in the form of STEAM_I:J:K where:

• I = Universe
• J = Lowest bit of Account ID (0 or 1)
• K = Highest 31 bits of Account ID

Modern Text:

Exists in the form of [C:U:A:I] where:

• C = Character representation of Account Type
• U = Universe
• A = Account ID
• I = Instance

64-bit Integer:

A bit more complicated, but you'll see this on the Steam Community and in the Steam Web APIs. This value is generated as a function of the 4 components described above using bit shifting and bit arithmetic. The function is:

((Universe << 56) | (Account Type << 52) | (Instance << 32) | Account ID)

64-bit Steam ID Components

Given 64-bit Steam ID: 76561197960361544    
---
Binary = 100010000000000000000000100000000000000010111011001001000
         ^^  ^^                  ^^                              ^
         ||__||__________________||______________________________|
         U AT       INSTANCE              32-BIT ACCOUNT ID
---
Account ID = Low 32 bits of 64-bit Steam ID
Account ID = 76561197960361544 & ((1 << 32) - 1)
Account ID = 95816
---
Instance = Next 20 bits of 64-bit Steam ID
Instance = (76561197960361544 >> 32) & ((1 << 20) - 1)
Instance = 1
---
Account Type = Next 4 bits of 64-bit Steam ID
Account Type = (76561197960361544 >> 52) & ((1 << 4) - 1)
Account Type = 1
---
Universe = Remaining (max of 8) bits of 64-bit Steam ID
Universe = (76561197960361544 >> 56) & ((1 << 8) - 1)
Universe = 1

64-bit Steam ID Legacy Text

Using the results of the 64-bit Steam ID to Components conversion, we can obtain the Legacy Text.

Legacy Text = STEAM_I:J:K = STEAM_1:0:47908
I = Universe = 1
J = Lowest bit of Account ID = Account ID & 1 = 0
K = Highest 31 bits of Account ID = (Account ID >> 1) & ((1 << 31) - 1) = 47908

64-bit Steam ID Modern Text

Using the results of the 64-bit Steam ID to Components conversion, we can obtain the Modern Text.

Legacy Text = [C:U:A:I] = [U:1:95816:1]
C = Account Type character (use lookup table on this page)
U = Universe
A = Account ID
I = Instance

Components 64-bit Steam ID

Given Components: 
Universe = 1    
Account Type = 1    
Instance = 1    
Account ID = 95816
---
64-bit Steam ID = ((Universe << 56) | (Account Type << 52) | (Instance << 32) | Account ID)
64-bit Steam ID = ((1 << 56) | (1 << 52) | (1 << 32) | 95816)
64-bit Steam ID = 76561197960361544

32-bit Account ID Estimated 64-bit Steam ID

Dota 2 (and probably other games) only identify a Steam account by its 32-bit Account ID. This is often incorrectly called the user's "32-bit Steam ID". There is actually no such thing as a 32-bit representation of a Steam ID. In fact, it's only 32 bits of the full 64-bit Steam ID!

So the only type of "conversion" that can occur between the 32-bit Account ID and the 64-bit Steam ID is an estimation by assuming values for the other missing components.

Note that this will not work if the Steam account has components other than U = 1, AT = 1, and I = 1.

Given 32-bit Account ID: 95816
Assume Universe = 1 (or 00000001 in binary)
Assume Account Type = 1 (or 0001 in binary)
Assume Instance = 1 or (or 00000000000000000001 in binary)
---
Bitwise OR against the assumed binary values from above.
64-bit Steam ID = Account ID | 00000001000100000000000000000001
64-bit Steam ID = 76561197960361544