Secondary Dominants
A V (or viiº) of a diatonic chord other than I
Most include at least one raised diatonic pitch (accidental)
This raised pitch is usually the 3rd of the secondary V chord or the root of the secondary viiº chord
This raised pitch is the Leading Tone to the tonicized pitch (secondary / temporary tonic)

The V7 / IV (in a major key) will not have a raised pitch
It will have a lowered 7th scale degree (the 7th of the chord)
The V7 / III (in a minor key) will not have any accidentals
Occasionally there will be more than one raised pitch
Sometimes both the 3rd and 5th of a secondary V will be raised in order to construct a major chord
V of iii in a major key
V of V in a minor key

Usually resolves to the tonicized chord (secondary / temporary tonic)
Functions like an authentic cadence
A Secondary V occasionally resolves up a 2nd (root movement)
Functions like a deceptive cadence