Straight bits
Short answer
A straight bit has no spiral, so it neither lifts nor presses chips. It leaves a rougher edge than a spiral, clears poorly in deep cuts, and is noisier. It is cheap, tough, and fine for shallow work and for material that might destroy a better bit.
What it is
Cutting edges running parallel to the axis of the tool, with no helix. It was the standard router bit for decades because it is simple to manufacture and it works fine in a handheld router where cuts are shallow and the operator controls the feed.
What the missing spiral costs you
- No chip direction. Chips are thrown sideways and rely on airflow to get out. In a deep cut they stay.
- Chopping rather than shearing. The whole cutting edge hits the material at once rather than progressively. That means a rougher edge, more noise, and more vibration.
- More tearout in wood. The chopping action is harsher on fibres.
Where a straight bit still makes sense
- Shallow, wide clearing where evacuation is not a problem and the edge will be sanded anyway.
- Reclaimed or salvaged wood that might have a hidden nail. Destroying a cheap straight bit is a better outcome than destroying a good spiral.
- Large diameters. Carbide tipped straight and profile bits are far cheaper than solid carbide spirals at big sizes.
- Profile cutters. Roundover, chamfer, cove and similar shapes are almost always straight or slightly sheared, not spiral, because the profile is the point.
- You already own it and the job is forgiving. A perfectly reasonable answer for learning.
If you use one
Keep the depth per pass smaller than you would with a spiral, keep the dust collection running so chips actually leave, and expect to sand the edge. Do not use one for a deep slot: that is the case where the lack of chip evacuation does real damage.