Gastric cancer (GC) is a multifactorial disease influenced by both environmental and genetic factors. It is the fourth leading cause of cancer-related deaths worldwide, with a median survival rate of less than 12 months for advanced-stage cases. The incidence rate increases with age, but approximately 10% of cases are detected in individuals aged 45 or younger. Early-onset gastric cancer is valuable for studying genetic alterations in carcinogenesis due to reduced exposure to environmental carcinogens. Carcinogenesis involves a multistage process characterized by progressive mutations and epigenetic changes in various genes. The most common classification of GC is the Lauren classification, which divides it into intestinal and diffuse subtypes. The intestinal subtype includes tubular and glandular elements, while the diffuse subtype consists of poorly cohesive single cells without gland formation. GC with signet ring cells is classified as a diffuse subtype. The five-year survival rate varies significantly, with Japan having the highest survival rate among developed countries. Key risk factors for GC include family history, diet, alcohol consumption, smoking, *Helicobacter pylori* infection, and *Epstein–Barr* virus infection. Family history is a crucial risk factor, with a three-fold higher risk for individuals with a family history compared to those without. Dietary factors, such as high intake of sodium chloride and N-nitroso compounds, can also increase the risk of GC. *Helicobacter pylori* infection is a significant risk factor, promoting epithelial–mesenchymal transition and further cancer progression. The World Health Organization (WHO) classification provides a detailed description of gastric tumors, including various subtypes of adenocarcinoma and other types of gastric tumors. Early detection through endoscopic examination is crucial for improving survival rates. Treatment strategies include surgical resection, adjuvant and neo-adjuvant chemotherapy, radiation therapy, and targeted therapies based on molecular characteristics of the tumor. In conclusion, GC is a complex disease influenced by multiple factors, and early detection and targeted treatments are essential for improving outcomes.Gastric cancer (GC) is a multifactorial disease influenced by both environmental and genetic factors. It is the fourth leading cause of cancer-related deaths worldwide, with a median survival rate of less than 12 months for advanced-stage cases. The incidence rate increases with age, but approximately 10% of cases are detected in individuals aged 45 or younger. Early-onset gastric cancer is valuable for studying genetic alterations in carcinogenesis due to reduced exposure to environmental carcinogens. Carcinogenesis involves a multistage process characterized by progressive mutations and epigenetic changes in various genes. The most common classification of GC is the Lauren classification, which divides it into intestinal and diffuse subtypes. The intestinal subtype includes tubular and glandular elements, while the diffuse subtype consists of poorly cohesive single cells without gland formation. GC with signet ring cells is classified as a diffuse subtype. The five-year survival rate varies significantly, with Japan having the highest survival rate among developed countries. Key risk factors for GC include family history, diet, alcohol consumption, smoking, *Helicobacter pylori* infection, and *Epstein–Barr* virus infection. Family history is a crucial risk factor, with a three-fold higher risk for individuals with a family history compared to those without. Dietary factors, such as high intake of sodium chloride and N-nitroso compounds, can also increase the risk of GC. *Helicobacter pylori* infection is a significant risk factor, promoting epithelial–mesenchymal transition and further cancer progression. The World Health Organization (WHO) classification provides a detailed description of gastric tumors, including various subtypes of adenocarcinoma and other types of gastric tumors. Early detection through endoscopic examination is crucial for improving survival rates. Treatment strategies include surgical resection, adjuvant and neo-adjuvant chemotherapy, radiation therapy, and targeted therapies based on molecular characteristics of the tumor. In conclusion, GC is a complex disease influenced by multiple factors, and early detection and targeted treatments are essential for improving outcomes.