Yes, that's a weed -- I'm pretty sure it's pigweed, which is a wild amaranth.

The smaller, round-leafed weeds below it are purslane, a portulaca that's edible and very nutritious.

This is what it looks like when a baby.

(I need to learn how to post more than one pic)

Definitely amaranth (aka Careless Weed, pigweed). It used to be a staple in the diet of native Americans. My desert tortoise loves them, especially when young.

There are two kinds of purslane. Horse purslane (Trianthema portulacastrum, larger rounded leaves) is not edible while common purslane is.