Socket 775: Up to 3.80GHz Quad Core

Socket 775, also known as LGA 775 or Socket T, is an Intel desktop CPU socket. This type of socket has been used since the Intel Pentium 4 generation up to Intel Core and Intel Core2. Speeds ranging all the way up to 3.80GHz and 4 cores or "Quad Core", with a Quad-Pumped Front Side Bus (FSB) of 533 MT/s up to 1600 MT/s.

Intel changed from Socket 478 to LGA 775 because the new pin type offers better power distribution to the processor, allowing the front side bus to be raised to 1600 MT/s. The 'T' in Socket T was derived from the now canceled Tejas core, which was to replace the Prescott core.

Used memory in combination with the socket 775 model was mainly DDR2. But since the memory industry so rapidly develops faster memory, the socket 775 model is also used in combination with DDR and DDR3 memory. Apart of DDR RAM, RIMM or RDRAM was also used in the beginning.
PC-800 RDRAM operated with a latency of 45 ns, which was more latency than other comparable DRAM technologies of the time. RDRAM memory chips also put out significantly more heat than SDRAM chips, necessitating heatspreaders on all RIMM devices. RDRAM includes a memory controller on each memory chip, significantly increasing manufacturing complexity compared to SDRAM, which used a single memory controller located on the northbridge chipset. RDRAM was also two to three times the price of PC-133 SDRAM due to a combination of high manufacturing costs and high license fees.
DDR-266 or PC-2100 DDR SDRAM, introduced in 2000, operates at 133 MHz  and delivered 2100 MB/s. Besides it was much less expensive,it also beat the speeds of PC-800 RDRAM, running at 400MHz and delivered 1600MB/s.

Since socket 775 there has been a change of placing the pins.
The land grid array (LGA) is a physical interface for microprocessors of the Intel Pentium 4 and AMD Opteron families. Unlike the pin grid array (PGA) interface found on most AMD and Intel processors, there are no pins on the chip; in place of the pins are pads of bare gold-plated copper that touch pins on the motherboard.

More information about socket 775's successor: http://www.socket1366.com/.