Tarantulas are probably the heaviest spiders that we have in Texas. Their bodies can be up to 1 1/2 inches long and they get even larger when you add in their legspan. Tarantulas create burrows either by digging one or using natural cavities under rocks, logs, etc. Sometimes they'll line the burrow with silk and add a few lines in front of the burrow to help detect prey that might wander past. Tarantulas eat things such as crickets, grasshoppers, beetles and caterpillars, so they are beneficial and help reduce pests that may be wandering around in your yard.
While the size and hairiness of tarantulas freaks most people out, they really are fairly harmless. Of course, they have fangs and are capable of biting, but typically their venom doesn't react with our body chemistry in a negative way. Tarantulas will let you know if you are upsetting them to allow you ample time to move away- they'll rear up on their hind legs and put their front legs into the air. If you see one do this, then leave it alone. Tarantulas may also brush their legs along their abdomen when threatened to brush off urticating hairs that can cause irritation to the eyes or skin.
The above photo is of Debbie, the Bird Eating Tarantula that I have in my office (I feed her mice, not birds). The quarter is in the photo so you can have a size reference.