As its name implies, it is a land grid array with 771 contacts.
The word "socket" in this instance is a misnomer, as the processor interface has no pin holes.
Instead, it has 771 protruding lands which touch contact points on the underside of the microprocessor.

The "J" in "Socket J" refers to the now-canceled processor codenamed "Jayhawk", which was expected to debut alongside this interface.
It is intended as a successor to Socket 604 and takes much of its design from LGA 775 and is almost pin compatible with LGA 775.
Socket 771 CPUs are rotated 90 degrees and have two pins swapped.
Small adapters developed by Chinese computer DIY enthusiasts are available to allow LGA 771 CPUs to be installed in LGA 775 motherboards, if the BIOS microcode supports them (but it is also possible to add supporting microcode manually).
As its name implies, it is a land grid array with 771 contacts. The word "socket" in this instance is a misnomer, as the processor interface has no pin holes. Instead, it has 771 protruding lands which touch contact points on the underside of the microprocessor.
The "J" in "Socket J" refers to the now-canceled processor codenamed "Jayhawk", which was expected to debut alongside this interface. It is intended as a successor to Socket 604 and takes much of its design from LGA 775 and is almost pin compatible with LGA 775. Socket 771 CPUs are rotated 90 degrees and have two pins swapped. Small adapters developed by Chinese computer DIY enthusiasts are available to allow LGA 771 CPUs to be installed in LGA 775 motherboards, if the BIOS microcode supports them (but it is also possible to add supporting microcode manually).
LGA 771