You can access it via IP address, but that poses a few issues. First, it's likely that your home connection has a dynamic IP address, which means that it could change if your connection goes offline, or your ISP decides to change it. Depending on your ISP this may not be very likely (I've had the same IP at my house since I moved in over a year ago) but it's a possibility. Therefore, giving clients the IP address to access a site is a little shaky. Plus, IP addresses are cumbersome to deal with compared to domains & subdomains.
Second, if you want to use subdomains to separate sites, the IP address won't get you there. You could separate sites using subdirectories in which case the IP would work, but that comes back to it being cumbersome.
Galaktican is right about needing to configure your router for port forwarding. Check the model number of your router and Google it - you can typically find an owner's manual or configuration information for your specific router this way. Port 80, as he mentioned, is the port for HTTP. Forward this port to your server's internal IP address.