Engagement Announcements and Parties are the more formal and better established tradition of Save the Date cards. A big difference is that for an engagement announcement and party, you don't need to have set the date of the wedding. As these parties are normally thrown for the couple, not by them, do your best to get a good picture of those two, be it the engagement photo, or something around that time frame. These are very often used on the card.
Engagement party invites from PurpleTrail are your chance to impress your guests, and prepare them for a party to celebrate the couple.