What is the difference between a PRF and a PRF+?
IKEv2 uses the term PRF to refer to a negotiated keyed random-looking function (for example, possibly HMAC-SHA256).  IKEv2 uses the term PRF+ to refer to a specific construction based on that underlying negotiated function, as defined in section 2.13:

prf+ is defined as:

prf+ (K,S) = T1 | T2 | T3 | T4 | ...

where:
T1 = prf (K, S | 0x01)
T2 = prf (K, T1 | S | 0x02)
T3 = prf (K, T2 | S | 0x03)
T4 = prf (K, T3 | S | 0x04)
...


The term PRF+ has no generally accepted meaning outside of IKEv2.