What Is the Difference between “Virtual Size” and “Actual Size” of a Transaction?

The actual size is the transaction's size in bytes as it appears on the disk. The virtual size (vSize) is the size used for fee calculation post-SegWit.

It is derived from the block weight calculation, where witness data is counted at a discounted rate. Virtual size is a weighted measure designed to incentivize the use of SegWit.