Software RAID implements RAID functionality in the kernel code (block device driver).
No special hardware is required, any hard disks (IDE, SCSI, USB, Firewire) can be used.
For a fast CPU, the load of running RAID is minimal (Software RAID takes advantage of MMX/SSE instructions)
Current Linux Software RAID is fast, full-featured, and reliable
Primarily aimed at preventing data loss, not preventing downtime
Usually has single point of failure (boot disk, IDE controller) but it's commodity hardware, easily replaced

Alternative: Hardware RAID

RAID controllers, external RAID boxes, Network Area Storage
High-end solutions provide both higher performance and higher reliability
Cheap RAID controllers lose to Software RAID in performance benchmarks (http://linas.org/linux/raid-reviews.html)
Mid-level solutions still have single point of failure (RAID controller), higher uptime only for high-end systems
When RAID hardware fails, what are the odds that you have a spare one in your closet?

Software RAID compares well unless high availability is the main goal and high-and redundant hardware is available. Software RAID is a better solution unless for a home user.

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Software RAID Software RAID implements RAID functionality in the kernel code (block device driver). No special hardware is required, any hard disks (IDE, SCSI, USB, Firewire) can be used. For a fast CPU, the load of running RAID is minimal (Software RAID takes advantage of MMX/SSE instructions) Current Linux Software RAID is fast, full-featured, and reliable Primarily aimed at preventing data loss, not preventing downtime Usually has single point of failure (boot disk, IDE controller) but it's commodity hardware, easily replaced Alternative: Hardware RAID RAID controllers, external RAID boxes, Network Area Storage High-end solutions provide both higher performance and higher reliability Cheap RAID controllers lose to Software RAID in performance benchmarks (http://linas.org/linux/raid-reviews.html) Mid-level solutions still have single point of failure (RAID controller), higher uptime only for high-end systems When RAID hardware fails, what are the odds that you have a spare one in your closet? Software RAID compares well unless high availability is the main goal and high-and redundant hardware is available. Software RAID is a better solution unless for a home user.