Self-propelled guns (SPGs), more often known as artillery, provide powerful indirect fire support with a very large range that can potentially cover the entire map. They stay safe by remaining well within friendly lines and provide support against tough stationary or slow targets. Artillery is just as good as the team. So if you (as arty) find yourself in a "bad team" or a team that only camps, you will not get the full potential to the powerful gun you have and support as much that you wanted to.
Almost exclusively, SPGs use HE shells lobbed in a ballistic arc to hit distant targets. The inaccuracy of their weapons, particularly at higher tiers, makes the splash damage caused by the explosion often the primary source of damage, with direct hits being crippling to the target tank. The heaviest SPG can weigh up to 115 tons. Speeds vary greatly and range from maximum speeds of below 20 kph to above 72 kph. Armor is usually negligible, and with a maximum armor thickness of 152 mm, SPGs are vulnerable to light and medium tanks. SPGs possess the most powerful weapons in the game, being able to deliver over twice the damage of the most powerful cannons available on similarly-tiered tanks. However, extremely poor accuracy, slow aiming speed, and a long reload time make their weapons only effective against slow and stationary tanks, or tanks moving in a straight line at a constant speed.
SPGs are capable of using Strategic View Mode (left shift), which gives them a bird's-eye view of the whole map, though due to screen size limitations you see only a small portion. Map navigation in this mode is very easy. Be aware that moving reticle sideways on the map will cause the vehicle to physically reposition itself, which will significantly reduce your firing accuracy. The reticle will still display as expected, but will vary in shape with the terrain. This eliminates the need to get reasonably close to the target to hit it.
SPGs work closely with scouts, using them to spot targets. SPG support is essential in a scenario in which smaller tanks are up against more powerful tanks, and can quickly turn the tide of a battle. SPGs are also one of the best weapons against opposing SPGs, able to fire blindly at potential enemy positions by spotting tracer fire or recently destroyed map-objects like trees or buildings.
SPGs should relocate after firing, and avoid predictable locations to prevent from being destroyed by counter-artillery fire. Good situational awareness is also essential to survive in an SPG. Noticing breaks in a defensive line early can allow an SPG to relocate before an enemy scout can arrive.
Another thing to remember with SPGs is that they have very high ammo-costs, for example a single round for an upgraded Hummel (and similar tiered SPGs) will cost about 1000 credits, which makes constant blind-firing and misses a costly endeavor.
SPGs appear as red or green squares on the mini-map and over target marker.