clock_t
Type capable of representing clock tick counts and support arithmetical operations.
This type is returned by the clock function of the <ctime> header to represent the number of clock ticks since the beginning of the program execution.
time_t
Type capable of representing times and support arithmetical operations.
This type is returned by the time function and is used as parameter by some other functions of the <ctime> header.
It is almost universally expected to be an integral value representing the number of seconds elapsed since 00:00 hours, Jan 1, 1970 UTC. This is due to historical reasons, since it corresponds to a unix timestamp, but is widely implemented in C libraries across all platforms.
转载：
下面转自：http://www.cnblogs.com/chenyadong/archive/2011/12/03/2274783.html
clock()返回从“开启这个程序进程”到“程序中调用clock()函数”时之间的CPU时钟计时单元（clock tick）数，而sleep(5)并不占用cpu资源，导致start1和end1返回的值一样。time(&temp)返回从CUT（Coordinated Universal Time）时间1970年1月1日00:00:00（称为UNIX系统的Epoch时间）到当前时刻的秒数。总之，用time_t计时才是人们正常意识上的秒数，而clock_t计时所表示的是占用CPU的时钟单元。