Invitation components...confused

I know it would seem simple but one really never takes notice of all the different pieces that are in a wedding invitation. I obviously know that I need the invitation, a response card and envelope and a reception card but what are inner envelopes, outer envelopes, etc. I know Ive seen these in invitations but now I cant figure this all out. Please help! Thank you!!

Im on a site and its asking this...

Extra Envelopes:
Specify additional envelopes desired in the boxes below.

Unlined Inner Envelopes - 15¢
Lined Inner Envelopes - 30¢
Non-Printed Outer Envelopes - 15¢
Printed Outer Envelopes - 30¢

To order 'extra' printed envelopes, you must first check the printed outer envelope checkbox above.

The outer envelope is what is actually stamped and addressed. The inner envelope holds the invite, and is placed into the outer envelope, and just has the names of the persons addressed on it.

Unlined inner envelopes are just plain envelopes.
Lined envelopes have a lining inside the flap, the lining can be color coordinated to your invite if you'd like.

Non-printed outer envelopes are plain envelopes.
Printed outer envelopes have your return address printed on the other side.

HTH!

ETA: Don't forget your response card envelopes, which you will want printed with your address!

The above response anwsered everything perfectly.

The inner envelopes are very specific of who is invitied. You will say the names so people know if their children are invitied or just adults. On my inner envelopes we were very informal. We wrote Uncle and Aunt not just the names. This is though informal, but more personnal.

If your doing the addresses yourself get extra envelopes!!!

Good luck