@Lizardfreak The term "instar" just indicates to how many molts it has completed. (Every tarantula, from the tiniest sling to an adult, is Xth instar, though usually we stop counting after the first few instars and just go by size.)
egg with legs (EWL): the stage immediately after eggs
1st instar (1i) sling: After EWLs molt, they are 1st instar. Some hobbyists refer to 1st instar slings are as "nymphs," although this term is more commonly used for immature insects. (At this stage, they look like tiny tarantulas but will generally not feed.)
2nd instar (2i) sling: After 1st instar slings molt, they are 2nd instar. At this stage, they normally begin to feed, although they may not yet take live prey. (This is the earliest instar when it is considered acceptable to sell a tarantula.)
sling: any baby tarantula (1st instar onward) that does not yet have adult coloring (usually smaller than 2" for non-dwarf species); most people don't track specific instars after 2i, because there is no practical reason to do so
juvenile: a sexually immature tarantula that has adult coloring but is less than 50% grown.
sub-adult: a sexually immature tarantula that is about 50-75% grown.
adult/mature: a sexually mature specimen, even if it has not yet achieved maximum size (females often mature long before they max out)