TeX Live 2018 is available for download now. It is also available on DVD from TUG and other user groups.
You can acquire TeX Live in many ways. For typical use, we recommend the first two:
The last general update of packages and programs for the DVD was made around 14 April 2018. However, we continuously update packages in TL as they are released to CTAN. Post-installation updates are done with the tlmgr program. tlmgr also provides many other TeX installation options and features, explained in the tlmgr documentation.
Although it is possible to update continuously, it is not necessary or even recommended. The set of packages and programs that were included in the official release will most likely suffice unless you know that you specifically need updates made since then. That set is also the only one which gets tested as a coherent release.
Older releases of TeX Live are available. You might find one useful if you need a release of particular vintage. There's no problem with having multiple versions of TL installed (in different directories); in fact, the default directory structure is arranged to facilitate this. After installation, just set your PATH accordingly.
This page is about acquiring the “original” TeX Live—the version of TeX released by the TeX user groups. If you want to get TeX as it is packaged by the various distros, see the information available from the particular distro you want. Some distro links.