Series Circuit and Parallel Circuit

On the left board a single loop of wire joins the battery to both bulbs. On the right board the wire splits into two branches, so each bulb has its own path back to the battery.

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In the picture

The battery on the left board pushes current around one single loop.
The first bulb on that loop glows dimly because it shares the one current path with the other bulb.
The wire between the two bulb holders carries the very same current through both of them.
The solder bead on the right board splits the current into two separate branches.
The bulb on its own branch gets the full battery voltage, so it glows brightly.

Examples

Old Christmas string lights wired in series: one burnt-out bulb leaves the whole strand dark.
The lamps in a room are wired in parallel: switching one off leaves the others shining.

Key takeaways

In a series circuit there is only one path, so the same current flows through every part.
In a parallel circuit each branch gets the full battery voltage and keeps working when another branch breaks.