Straw is basically the leftover stalk after they harvest a grain. Could be almost anything. Weeds/feed grasses etc. will always be mixed in to a greater or lesser extent. Some claim that you need straw for mulching because hay has seeds: this is based on the idea that the seed head has been harvested, but it turns out to not always be correct.

Alfalfa has a cloverish look to the leaves. It's a legume, so it fixes nitrogen. It is the basis for many feed stocks- rabbit and horse food are both normally alfalfa based.

On a plant it would look like this:

Here is a link that might be useful: {{gwi:293410}}

Thank you for the replies. So I guess the straw isn't alfalfa.

Straw is usually the stalks left over after oats or wheat have been threshed off the plants. Al Pablo indicated, not all straw is free of seeds. A lot depends on how it was harvested and the efficiency of the machinery. Some if it is likely to have quite a bit of seed, but any seed that manage to survive and come up from that is easy to pull up or deal with.

I really like alfalfa, it is a good natural, slow release nitrogen source and is also supposed to contain a natural growth harmone which might possibly benefit your plants. My sister in law who raises horses swept up, and filled several feed bags with broken up bits of high dollar alfalfa hay. I have added it in with other materials to prep a row for planting in my garden, and things seem to be growing great.

I do not know if you can tell much about it from {{gwi:258834}} of it or not. Most alfalfa hay is cut and baled before it seeds out, I have never, ever had any problem with seed sprouting from using it.
Just my.02
Bill P.