The web server is the computer through which your customers will access your web site. Therefore it must be permanently connected to the Internet and available 24/7.
In most circumstances, your micommerce web sites are hosted on either a shared or dedicated server in our ISP partner's data centre. This is not however a prerequisite, as you are free host it on your own server, with an alternative ISP or internally.
In addition to providing you with your ecommerce web site, the web server acts as a 'mirror' for all your internal micommerce data. As each transaction is received, or a change is made, it is 'synchronised' with the opposite database. E.g. If a price is changed internally, it will be automatically 'synchronised' with the web site, so that the new price shows immediately. Equally, if an order is placed online, that order will be automatically 'synchronised' with the local micommerce database, so that it can be processed along with those others from each of your other channels.
