If hosted at home on one of your computers than the problem is most likely that ports are not forwarded out of your network correctly. It could also be that people are trying to connect from outside of your network (i.e. a friends house) but you gave them your internal MC server ip instead of your WAN ip address.
LAN ip looks like this..... ex ... 192.168.1.1 or possibly 10.0.0.45 or similar
WAN ip looks similar to this ex.... 66.94.230.43
Try this if you know that you did not give your friends the wan ip address and they are connecting from somewhere other than the same network that the server is on:
Go here:
http://www.whatsmyip.org/ Get the ip from the top of the page and give it to your friend. Have him use it as the server ip with the port you want to use. See if that works.
If not. Then you still will need to forward ports.
It is easier for you to follow along with a guide for the router that you have at home, then for me to tell you how to forward ports.
Go to google and type in your routers name and then port forward minecraft. Try to find a guide for it.
For example if my router was a linksys e2000 or something like that then i would search linksys e2000 port forward minecraft
