An article by Rob Andrews at sponde.com describes altars in the following way:

In a general sense, altars have been and are used as platforms on which to give sacrifices and pour libations for any of a huge array of reasons. Certain altars may be set aside purely for devotional purposes, whereas others have been tied to oracles, healing miracles, or other special functions. As events surrounding a given altar, deity, or community progress, a given altar's design, use, and purpose may evolve. However, once something is given in sacrifice, it must be given over completely and not reclaimed.

My personal altars serve as devotional spaces. I currently have two set up, with plans for more in the works. The first is my main altar: I use this altar for my Hestia devotionals, and also as my general devotional altar for all of the Olympians. The altar, when possible depending on our current apartment, sits in the corner of our main room, and is just about as close to the center of the house as possible (as shown in the photos of the altar). Our current apartment doesn't really allow for that, so instead I have it in the main room between two windows. It consists of a table gained from an old friend that was once an outdoor garden table with a marble top and Greek/Roman style art around the stem. It holds my Hestia candle, my incense burner, various representations of various deities and a few other things. My second altar, and my first individual altar, is devoted to Aphrodite. It sits on the top of a tall, standing jewelry box with a statue of Aphrodite, a crystal dish, a rose-scented candle that a friend gave me, and a fabric rose. On occasion, an offering of fresh flowers (often red roses) are laid out on this altar.

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Main altar
Aphrodite's altar