The two terms mean the same thing, it's a small piece of knitting worked in the suggested pattern to ensure you have the right number or rows and stitches per 4 inch or 10cm square. If you have more stitches per 4 inch, try a larger needle, if you have less stitches per 4 inch try a smaller needle. I usually make a square that's approximately 6 inch or 15cm square, a larger piece gives me a more accurate result when I am counting my stitches per 4 inch/10cm square. If you don't knit a swatch then your finished object (FO) could end up being a lot smaller or larger than you wanted or anticipated. With a hat, that's maybe not such a problem as it'll fit someone, but a sweater could be a lot larger or smaller than you need. It could be too long or too short and end up wasting your time and money. If the front of your sweater has 100 stitches, at a gauge/tension of 20 sts = 4 inch it will measure 20 inches in width making a 40 inch sweater. But, at a gauge/tension of 18 sts = 4 inch it will measure 22 inches in width, making a 44 inch size sweater. If your sweater works out at 4 sts per inch, the end result would be 50 inch chest. That's quite a difference in terms of fit and possibly yarn usage. Always block your gauge/tension swatch after you've finished knitting and cast off the piece. You need to block it the same way you intend to block the finished item, whether that's wet blocked, steam blocked or spray blocked and pinned to shape. Your square in lace will typically be larger than a stocking stitch square because of the holes/eyelets.