Spawn point

Spawn points are the location on a map where a player originally starts. Every map has 2 spawn points: a red and a blue one, one for each team. Each spawn point is surrounded by a box of the corresponding color, where conj cannot be placed, and a little platform 2 blocks wide of the corresponding color that cannot be destroyed. Spawn points remove blocks within their colored area, including on certain flag maps that move their spawn points after a flag is captured.

Re-spawning
While players always start off a map on a spawn point, only in Flag maps do players ever re-spawn there. In VIP maps the player will spawn at the center of the VIP, and in Domination maps the player never re-spawns. After a player re-spawns, they are invincible for 4 seconds, even if they move out of the spawn point (this it to prevent spawn killing).

Spawn Killing
Spawn killing is killing some when they respawn before they have time to understand what's happening. Spawn killing can be done in groups, and is often annoying as it can distract player from the real goal (which isn't fighting of spawn killers). While spawn killing is much less practical due to an update (before V1.26), that grants you "invincibility" after being re-spawned. However if the spawn killers waits far enough away, they can "spawn kill" after the invincibility period wears off. It is highly advised to ignore spawn kills when possible, and just walk past them to your objective (using your invincibility from re-spawning).

Spawn Blocking
Spawn blocking is similar to spawn killing in that it's intended to be a distraction / give the player an advantage, but when done right it can be much more effective. This is done by finding an area on the map (usually near the spawn) that is easy to block and filling it with conj, or simply placing conj around the actual spawn point itself. While "spawn blocking" is traditionally a Flag map concept, similar logic can be used in many situations. Spawn blocking is essentially taking advantage of choke points and filling them with conj (often giving yourself a good firing position), and this can be done on VIP and Domination maps alike.

Trivia

 * Before an update (before V1.10), you were able to spawn conj inside the current spawn area (there was no colored area), there were no indestructible blocks, and when a spawn moved it did not remove blocks around it (causing players to get stuck). A spawn was only marked with a smoke grenade without any of the other things we have with it today.
 * Before an update (before V1.26), you did not have any "invincibility" after being re-spawned.