overclock means to run processor at higher clock rate than default
if you have good motherboard for it (mine's o k) you can't make any harm (actually you can, so you have to be careful
)
you can change it in BIOS, BUT you have know that by increasing cycles you have to increase voltage in power supply and be aware of temperature. monitor it so you can see if you need some new cooler or just opening the case, you know what i mean
if you overcock your cpu too much, you can experience freezes and sometimes you have to restart CMOS by putting away batterie on MB, or on some mother boards there is little jumper for it
oh and that "+" you can find in AMD processors, becouse they define processor speed somehow differently and 2600 is not real speed of it (iam not expert on this)