The following was my reply:
Altars are a very personal thing and can be used for many purposes. Mainly though they are a place to pray, do spiritual work, meditate and honor the deities to whom you feel close. There is no general Pagan requirement for an altar, though certain traditions may prescribe certain things such as having the altar face east or placing items relating to Goddess on the left side and items related to God on the right. Altars can and are made anywhere, from a table, to a bookshelf, to the inside of an Altoid can--and yes, that is a current DIY trend--mini, portable altars meticulously constructed inside tiny mint cans. The only thing that I would say is absolutely common among Pagans regarding their altars is that they are regarded as a sacred place and reserved for spiritual devotion and practice--in other words, no putting coffee cups and junk mail on the altar table.
Some common things you will find on Pagan altars are: an image of the deity or deities being worshiped, items that represent that deity (such as peacock feathers for Hera or a quill pen for Thoth), a candle and/or incense, divination tools such as Tarot cards or runes, and a spiritual journal or "Book of Shadows"--though not all Pagans call it that.
People's altars tend to grow and change over time and reflect their current place in their spiritual path. For example, mine started out as very Wiccan in form and has progressed with my spirituality to now reflect my devotion to a Hellenic path.
Some Pagans have many altars in their homes dedicated to different deities and/or their ancestors, while others, limited by space or time, may only have a candle in a special reserved spot on their dresser that acts as their altar. Some also dedicate altars to specific purposes rather than deities. For example, while I have a private altar dedicated to Theoi in general, my fiancee and I also have an altar in more public view devoted to home, hearth, and love.
Humans seem to me to be natural altar makers--a process that is apparently so deeply ingrained in our collective subconscious that we do it without even realizing. If you look at most people's refrigerator doors, for example, you will usually find a natural altar to the family with pictures of loved ones, drawings, magnets that represent the personality of the family, etc. Or often people build an ancestor altar in their hallways by hanging pictures of their family members that have passed and, every time they walk by those pictures, their heart reaches out to them in love and respect.May we all come to acknowledge the various altars in our lives--whether outward or deep within us, they enrich our spirits in ways we can only imagine.
This reaching out--a spiritual touchstone--is really the core meaning of any altar, whether built specifically with that intent by a Pagan using candles and feathers, a Buddhist with a singing bowl, a Catholic with a Saint's icon, or anyone else with a life-decorated fridge door.
Blessed by the Mystery,
*The above picture is the "Home, Hearth and Love" altar mentioned above. It is dedicated to Hestia, Hera and Zeus