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Linux uses a program known as Udev to manage devices (dev is short for "device"). The /dev directory is a root directory for devices.
/dev/sda is SCSI Device A (or SATA Device A). This would be the first hard drive in the system. Following SATA/SCSI drives would be /dev/sdb, /dev/sdc, etc etc.
The number following /dev/sda is the partition number. Typically /dev/sda1 is the boot partition. This partition is usually no larger than 128KB in size and is where the bootloader and kernel reside to boot the system. /dev/sda2 would be the swap partition and is typically the same size as the amount of physical RAM you have in the system. /dev/sda3 would be the root partition and usually occupies the rest of the drive.
A typical partition table on one of my Gentoo systems looks like such -