Urine formation involves following three steps:
1) Ultrafiltration: It occurs in renal corpuscle (glomerulus & Bowman's capsule). It is the filtration under pressure hence called ultrafiltration. All the substances in the blood gets filtered except blood cells and plasma proteins thus the filtered formed in ultrafiltration is also called deproteinized plasma.
2) Selective Reabsorption: It mainly occurs in PCT and in low in Loop of Henle and DCT. In this step it works same as it says. The useful components which are filtered in ultrafiltration are reabsorbed in this step. 100% of glucose, 95% of water, 70% of Na+ ions are reabsorbed in blood.
3) Tubular Secretion: This step is reverse of selective reabsorption. The harmful substances which are not filtered in ultrafiltration are secreted in the filtrate by this step.
All the three steps occurs in different parts of nephron (ultrafiltration in renal corpuscle and selective reabsorption and tubular secretion in renal tubule)
When the fluid passes through the above three steps then finally the urine is formed at the CT and CD of nephron.