An SMS gateway is the service that accepts messages from applications (through an API or a web panel) and hands them to mobile carriers for delivery, returning statuses on the way back.
A gateway sits between your software and the carriers. It exposes an API or a panel on one side and holds connections to operators or aggregators on the other. In between it normalises numbers, applies sender rules, prices each message, queues and paces traffic, and collects delivery receipts.
The differences between gateways are in what they ask of you (documents, contracts, minimums), how they price (quotes and tiers versus a published card), how they route (one measured route or the cheapest available) and how honestly they report (per-message reasons or campaign totals).
At SMSMeteor: the gateway is the whole product: an email address opens the account, the rate card is public, and one route per destination is reviewed daily. Read how it compares.