For the Engrailed Cross (French/heraldic: Croix Engreslée), the obvious feature is the rugged edges of each arm. 'Engrailed' simply means dimpled with concave curves, similar to the convex curves seen on the edge of a scallop shell (see Compostelan Cross). Grail reminds us of the Holy Grail; the cup used by Jesus at the Last Supper (see Communion Cross).
The word 'engrailed' is distantly related to grill, implying protection. (Actually, grill is not such a neat analogy for Christians, since although a grill can protect something precious, the precious gift of salvation is not locked away but freely available. Indeed, the cross can make us free.)
The pattern also looks a bit like waves on the ocean; hence an alternative name: Wavy Cross. (See also another cross we call a Wavy Cross.) Other names include Indented Cross (French/heraldic: Croix Denchée).
The feature of the Engrailed Cross for Christians is its spikes, which represent the thorns piercing Jesus' head during his Crucifixion (see Crown of Thorns Cross).
A similar heraldic cross, but with the spikes pointing inwards, is called the Invected Cross. For Christians, this is known as the Paternoster Cross.
A heraldic cross with the spikes pointing both inward and outward, like a jagged saw-tooth edge, is called an Indented Cross (French: Croix Vivrée). Where the teeth angles are more acute it is called a Dancetty Cross (French: Croix Écotée). Both the Indented and Dancetty Crosses look similar to the Flaming Cross.