Skullcrushers - AKA Lying Triceps Extensions..........usually done with an E-Z Curl bar or a straight barbell.
Can also be done with two dumbells simultaneously,or one dumbell,usually to the opposite side of the head...............all are performed while lying on a flat bench,decline bench or even the floor.
Triceps Extensions - AKA French Press.............Usually done seated with an E-Z Curl bar,but can be done standing or on an incline bench.
Also performed with a single dumbell held with both hands usually seated also, or a single dumbell one arm at a time (one-arm dumbell extensions),alternating arms.
Both exercise can be done with a low cable also.