In summary, you will generally receive the following from the author:
The link to a sign-up page
The link to download the book. Alternatively, it could be emailed to you.
After release, the link to the live book for your review on Amazon, Goodreads etc.
A reminder to post your reviews after the due date
The sign-up page
The first thing an ARC reader could expect is a link to a sign-up page (example here), the purpose of which is twofold:
To provide you with information on what the author expects
To let the author know the extent of your online presence and where you will post your reviews (if you choose to)
It will usually contain the following:
Book cover
Book Blurb
Tropes
Trigger Warnings
Release date for the book
When review should be posted, such as within a week of launch
Preferred Sites such as Amazon or Goodreads
Some may have requests such as
If your review is under 4 stars it would be a great help if you delayed submitting your review for 2 weeks
If you choose not to review, that’s okay too. If you DNF (Did Not Finish), I'd appreciate you letting me know why.
In effect, the writer is asking for a head start, but this is a request only and up to your discretion. The most important thing is that a review be honest which is what gives the ARC system legitimacy.