This is a big distinction worth knowing. A primary ticket seller is a website, like Ticketmaster, that works directly with the event to sell its tickets. That means they work out a deal based on a price that works for both. From a buyer's perspective, this is the most secure way to buy, with tickets right from the seller, meaning you get them officially. The downside is that the price might already be high, despite no official markup, since the seller and event organizer decide this and control the first batch of tickets.
Secondary ticketing sites are those that re-sell tickets for an event. This is a great option if the event is a sell-out and you still want to get a ticket or two. These will be charged at a markup, which will cost you more than the face-value ticket price in most cases. However, you can sometimes find that these are actually cheaper than primary pricing. In most cases, these come with guarantees, so even though you're buying secondary tickets, they should still be secure, get to you in time, and actually work on the day.