The Shop is a virtual in-game system, giving you upgrades for your stats and consumables for Tix, which is earned by killing NPCs or enemies. The Shop Button is located at the bottom left corner, below your stats.
The Shop GUI:
If you click on it, a GUI will appear, with tabs labeled +Skillz, +H4X, +Toughness, +Resistance, +Speed, +Health, Hybrid Offense, and Consumables. In each tab, there are various upgrades that you can choose, but sometimes, you need to have other upgrades, in order to buy another upgrade. For example, to get the Bluesteel Blade, you need to have 400 Tix + Normal Sword Upgrade.
If you have a special upgrade (such as the Ghostwalker or DDos) or a consumable, you can use the keys Z,X,C,V,F in order to use them. The Ghostwalker can increase your speed by 20% for 5 seconds. The DDoS decrease enemy's Resistance if they get hit by a projectile by 40% for 4 seconds. They may cost a lot in total, but they can increase your stats more.
Consumables are potions that you can use that can give you helpful effects and can be activated during battle. Consumables are cheap, but they have only one dose.