These terms cover a wide range of activities. If your work is unpublished, be very cautious about agreeing to have it published first, or only, online or as an ebook/print-on-demand. This is an easy market for vanity publishers (the production, marketing and distribution costs of making ebooks and running off print-on-demand copies are very low). Even if you are not being asked for money, proceed with great caution.
Such companies often have little incentive to sell or promote your work (which may well only be listed on the publisher's website and in its catalogue), and may be earning their money in other ways, e.g. selling advertising on their website; or by buying up an author's other rights, so if a work takes off in future they are guaranteed a percentage of the earnings.
Be realistic about who your readers might be. How many people do you know who have sought out such websites? And of them, how many have thereby come across a book they had not heard of before, bought it and read it? If the site is just one of many offering a random selection of titles covering a miscellaneous range of subject areas (not always by top quality writers), ask yourself: is that really the best home for my work?
If you go ahead, limit the rights you are granting to ebook rights only; and make sure that you can terminate the contract after, say, two years. A better alternative might be to set up your own website and sell copies of the book (self-published in printed or electronic form) yourself.