The PSK is derived from the passphrase using PBKDF2 key derivation function with SHA1 as the pseudo random function. The passphrase is an 8-63 character ASCII encoded string.
PSK = PBKDF2(PassPhrase, ssid, ssidLength, 4096, 256)
The PSK is 32 bytes (256 bits), often displayed as 64 hex characters.
According to the 802.11i specification:
A pass-phrase is a sequence of between 8 and 63 ASCII-encoded characters. The limit of 63 comes
from the desire to distinguish between a pass-phrase and a PSK displayed as 64 hexadecimal
characters.
So the difference is just to distinguish a 64 hex character PSK from a 8-63 character ASCII passhprase.