Your question is extremely vague, but I'll make some assumptions and offer a suggestion.
If you're talking about monitoring network bandwidth usage on a linux system, IPTraf is very nice, and should be available for installation through your distro's package repository.
If my answer misses the mark, please edit your question in include what OS you're running and what type of "data transfer" you're looking to monitor.
Juan as others mentioned your question is very vague, but I find people often are looking for something like iftop to show the current input and output traffic rates and where they are going. If it's not installed you can get it with sudo apt-get install iftop. If that or the other answers don't suit your needs you probably need to me much more explicit in your question.
Your question is extremely vague, but I'll make some assumptions and offer a suggestion.
If you're talking about monitoring network bandwidth usage on a linux system, IPTraf is very nice, and should be available for installation through your distro's package repository.
If my answer misses the mark, please edit your question in include what OS you're running and what type of "data transfer" you're looking to monitor.
The answer for a single host is similar to the answer for many hosts. Setup SNMP on the host, gather the data using your favorite tool.
Juan as others mentioned your question is very vague, but I find people often are looking for something like
iftopto show the current input and output traffic rates and where they are going. If it's not installed you can get it withsudo apt-get install iftop. If that or the other answers don't suit your needs you probably need to me much more explicit in your question.