brk(2) sets the end of the data segment to the value specified by end_data_segment, when that value is reasonable, the system does have enough memory and the process does not exceed its max data size (see setrlimit(2)).
sbrk(2) increments the program's data space by increment bytes. sbrk(2) isn't a system call, it is just a C library wrapper. Calling sbrk(2) with an increment of 0 can be used to find the current location of the program break.
sbrk(2) is not defined in the C Standard and is deliberately excluded from the POSIX.1 standard (see paragraphs B.184.108.40.206 and B.8.3.3).