On the Subject of Which is What..
Elements Vs. Elementals
Elements are the actual spirit of the element, like Earth Spirit, Air Spirit, Fire Spirit, Water Spirit.
Elementals on the other hand are creatures that dwell in that realm. Some common ones are as follows: Earth - Gnomes, Air - Sprites, Fire - Salamander, Water - Sea Nymphs. You can research these and you may use whatever you are most comfortable with if you choose to work with elementals at all. Keep in mind, however, that certain Celtic Traditions do not consider fire an element, and that they believe the element of fire was brought to the Celtic lands by the Gauls.... The traditional elements are Land, Sky, and Sea.
Invoke and Evoke
Invocation; Invoke is to actually invite the spirit into your body. The reason for this is to do workings in that Goddess/Gods honor to aid you (normally amongst most pagans this is usually only done with Gods or Goddesses). You can also do this to act out stories Such as the Oak and the Holly King's battle at Midsummer and Yule.(See the Wheel of the Year section for details) Another example would be Drawing Down the Moon. Some view this as 'dangerous', but it is not the same as possession, and many non-pagans will argue this point forever. The fact of the matter is, it is far LESS dangerous to invoke a GOD/DESS than it is to use drugs or alcohol in ritual. The only substance normally used in my practice is Wine at the "Cakes and Ale" portion of the ritual, ('Ritual' section coming soon..), and this is not 'getting drunk' this is merely a small goblet, or chalice of wine, not the whole friggen bottle, and you don't drink the entire chalice-full either, because an offering is usually made with the cakes and ale to the God/dess invited to the ritual (in this case that would be evocation, if you are simply 'inviting' them... (For information on Esbats, see the Celtic Tree Calendar section for more details)
Evocation; Evoke is to call the God/Goddess/Spirit to attend the rite you are performing. You Evoke the God and Goddess when you perform any kind of circle or rite usually, You evoke the elements/elementals at each of the quarters, God and Goddess should always be invited to a circle, IMO, because you would always invite a guest of honor to the party, wouldn't you?