System.schedule in Apex scheduler in Salsforce

Sample code:

SampleSchedulableClass obj = new SampleSchedulableClass();

String cron = '0 59 * * * *'
System.schedule('Testing', cron, obj);

The above code executes SampleSchedulableClass for every one hour.

Syntax for Cron:

Seconds Minutes Hours Day Month Week Year

Seconds : 0 - 59
Minutes  : 0 - 59
Hours     : 0 - 23
Day        : 1 - 31
Month    : 1 - 12
Week     : 1 - 7(Sunday is first and Saturday is last)
Year       : upto 2099

Year is optional.

Expression	Description
0 0 13 * * ?	Class runs every day at 1 PM.
0 0 22 ? * 6L	Class runs the last Friday of every month at 10 PM.
0 0 10 ? * MON-FRI	Class runs Monday through Friday at 10 AM.
0 0 20 * * ? 2010	Class runs every day at 8 PM during the year 2010.

? - No value
* - All values
L - Last
W - Nearest weekday

Cheers!!!