Actually, the sprue ( or frame) is the path the plastic takes to the part. And the gate is the connection between the sprue and the part itself. The plastic is heated to a semi-vicous state, but it's still quite thick, which is why it takes tons of clamping preassure to keep the mold closed. The plastic is then cooled to almost solid within seconds. This goes back and forth for sometimes 30 times a minute. It's like slowing from 13,000 mph to 2 mph in 7 minutes.
As pointed out, the "sprue" is the large opening where the molten styrene is injected into the mold. The plastic travels along the "runners" to reach the part cavities. It enters the part cavity through a narrow opening called the gate.
Not sure where the term frame comes from, but it must be used to describe the complete runner system that holds the parts.