Among Pagan or Wiccan symbols, the most commonly seen one is the pentagram (though most Pagans or witches actually call it a pentacle instead). It's the infamous 5-pointed star inside a circle that seems to get so much negative attention, though most people don't have a clue what it means or what it symbolizes. It has come to be a relatively universal symbol to represent Paganism and witchcraft, though not everyone's path looks at the pentagram the same way.
A more general meaning for the pentacle is that the 5 points each represent one of the 4 elements, and the 5th (top) point stands for one's own spirit. The circle links them all together, illustrating how we are all part of one unity. Looking at a numerological point of view, the 5 points of the pentagram stands for just the element of Earth and can be used to represent the Earth on an altar.
Of course, there are many other Wiccan symbols that you may come across besides just the pentacle. Here are a few more:
The ankh is an Egyptian symbol that stands for "everlasting life". Heiroglyphic images of the ankh often show it as an object, held by the loop like a key. Though not originally a symbol that marked any particular spiritual path, it's a common symbol among Pagans to show of their beliefs without the stigma of the pentacle.
You may recognize this from the TV show "Charmed". Before it was part of pop culture, the triquetra was a Celtic symbol that represented the sacred number 3. Three was the number of elements in their view: sky, land and sea. You can even see this sign occasionally from Christian sources, representing their holy trinity (sometimes with a circle). Because of that, it can be a good symbol to wear when you aren't fully out of the closet yet.
The triple moon symbol is the only one that is more or less an actual image of something. Referencing the same sacred 3 as the triquetra, the triple moon shows the 3 main phases of the moon: waxing, full and waning. It also represents the 3 phases of the Goddess as well, in Her maiden, mother and crone aspects. So not only is it important for the 3, but also as a Wiccan symbol of the sacred feminine.
And now for a couple less commonly seen symbols. First, the septagram. This is a seven-pointed star, drawn in the same style as the pentagram just with a few more points. Unlike the association with Earth that the 5-pointed pentacle has, the septagram has a more spiritual meaning as the number 7 is connected to the astral or fairy realm. You can also view the points as representing the 4 elements, and then also above, below and within. It's sometimes drawn with a circle joining all the points, but not always.
Lastly, the hexagram. This one is not that well-known at all but I happen to like it so I'm including it. It's sometimes called a unicursal hexagram because it's a 6-pointed star that can be drawn as one continuous line rather than using 2 triangles (like in a Star of David). And like the other star symbols, the number of points has it's own meaning. In this case, 6 stands for balance and continuity. Aleister Crowley used it to represent balance between the physical world and the astral world.