A baby that is SGA is a baby whose weight is below the 10th percentile for gestational age (by dates, and/or by assessment). SGA babies can be term, preterm, or postterm.
IUGR babies are ones who haven't grown in utero at the rate they'd be expected to. A lot of IUGR babies are SGA, but not all.
IUGR can be symmetrical -- or maybe a better way to put it would be that ALL of their measurements (height, weight, and head circumference) are less than what you'd expect for that particular gestation. Or it can be asymmetrical -- say, two of the three measurements are normal, but the third is less than expected. Say their HC and length are normal, but the weight is low. Those babies look really wasted and malnourished, and a lot look like their heads are too big for their bodies.
This is a really short answer; this post could go on and on, and maybe there's someone who knows a better way to explain it than this. But either way, I hope it helps.
IUGR babies are usually associated with something that prevents them from growing well, such as PIH, poor placental perfusion, TORCH infections. SGA means you grew a little baby lol That is the simplest way to put it. But, and IUGR can be SGA...which is confusing lol My one daughter was SGA...she was just little, I didn't have any problems, she didn't have any problems. My best friend had an IUGR baby because she was hypertensive for most of her pregnancy and her placenta wasn't doing a good job.
But I have to say....IUGR babies are one of my most favorite ones to take care of lol The best one was a 34 weeker that weighed 900 gms...she was awesome....little but thought she was a big baby lol We also had a 28 weeker that weighed 450gm...she did okay, she had a few issues, but by far did well. We didn't think she would be alive after a few days, but she surprised us all.