When people say "backorder expired domains", what they are talking about are domains that reached pending delete state. Pending Delete Domains will definitely be deleted and can be registered by anyone.
If you find an expired domain name you want, you can place a backorder
for the domain with a drop catching / backorder service.
These services are specialized in automatically registering expired domains as soon as they become available again.
At this point anyone can register the domain name after the first come first served principle.
Because of that, the fastest service wins the domain. We are talking seconds if not milliseconds here.
That being said, you can of course try to hand register the domain name. If no one fancies it you can get it the regular way by just registering it with your preferred registrar. That is of course always a gamble, because you never know if someone else wants the same domain and is using a backorder service to get it! In that case the other guy is probably going to get it.
So if you really want the domain, you should consider placing backorders with one or multiple backorder services. You can place a backorder on multiple platforms, because for most of them you only pay if they are successful! That means if you get the domain, you only pay once. If you not win the domain, you pay nothing! You should not use services that charge you even if they fail to register the domain name for you.
Another thing to consider is that if multiple persons place a backorder for the same domain at the same backorder service and the company wins the domain, the domain most likely will be auctioned off. How exactly that works is different for every service. Sometimes there is no auction and the first person placing a backorder receives the domain. Some services hold a private auction only for the people that placed a backorder for the domain, but the majority of services now will hold a public auction, so anyone can join in and bid for the domain name. You should check how exactly it works for the service(s) you choose.
Backorder TLDs: .eu .at .be .ch .cz .de .es .fr .it .li .me .nl .nu .pl .se .uk
Backorder TLDs: .com .net .org .biz .info .mobi .co .me .us
Backorder TLDs: .com .net .org .biz .info .mobi .eu .travel .asia .pro .ac .ag .am .at .be .bz .ch .cl .co .cx .es .fi .fm .fo .fr .gd .hk .id .in .io .it .jp .lc .li .me .mn .nl .nu .pe .pm .pt .pw .re .ro .sc .se .sh .sk .tf .us .vc .vg .wf .yt (284 ngTLDs)
Backorder TLDs: .com .net .org .biz .info .mobi .aero .travel .asia .pro .ac .ag .bz .cc .ch .co .fm .io .it .lc .me .mn .nl .nu .pw .sc .se .sh .uk .us .vc (272 ngTLDs)
Backorder TLDs: .com .net .org .cc
Backorder TLDs: .com .net .org .biz .info .cc .us
Backorder TLDs: .cz
Backorder TLDs: .nz
Backorder TLDs: .au
Backorder TLDs: .ro
Backorder TLDs: .pt
Backorder TLDs: .fi .nu .se
Backorder TLDs: .com .net .org .info .mobi .travel .pro .ac .co .fm .fo .gd .io .pw .sh .vg (322 ngTLDs)
Backorder TLDs: .com .net .org .biz .info .mobi .name .asia .pro .ac .ag .bz .ca .cc .cn .co .io .lc .me .mn .mx .sc .sh .uk .us .vc (23 ngTLDs)
PreRelease, Closeouts, Expired Auctions
There is another common way of buying expired domains. This process has different names: PreRelease, Closeouts,
Expired Auctions or whatever the company offering these services wants to name it.
Here the domain will be auctioned off, before it reaches pending delete state! In this case the old domain registrar of the domain still has access to the domain name and can renew it if he wants.
Most of the time the old "owner"/"registrant" of the domain name has no idea about this auction or doesn't care, however if he knows and cares, he can still renew the domain himself and that has priority over the auction! There is usually an overlap of days the old owner can renew the domain and the auction process, so you might win a domain in this auction, just to find out that the old owner renewed it and you will not get it. In this case you usually do not have to pay for it or get your money back.
Domains that are successfully auctioned off are just renewed and moved to the auction winners account. If no one bids on the domain name, the regular expired domain process is triggered and the domain goes into pending delete state with the process mentioned above happens.
It is also important to mention, that not every expired domain is auctioned off before it reaches pending delete! Only domains registered with a registrar that participates in some kind of pre-release auction process will show up here. There are a lot of registrars that do not auction off expired domains! You will only find out about these domains when they reach pending delete and automatically show up in the pending delete droplist.