These are the current modes in Bug Heroes 2 (Single Player)
Endless Mode is a mode where the task is to survive for as long as you can against endless waves of enemy bugs, completing in game missions along the way. You can earn diamonds and stars from this mode, you can actually earn more diamonds from this mode than skirmish mode.
Skirmish Mode is a mode where you control two heroes and fight against two AI controlled heroes. The first team to destroy their opponent's stash wins. You can only earn diamonds from this mode, where easy grants one diamond, medium grants 2, and hard grants 3.
Mission Mode is a mode where you have to complete missions that earn you stars and diamonds. Stars are used to buy global upgrades and get new heroes. When you get stars the total amount remains the same even if you buy global upgrades. For example: If you have 5 stars and you buy a global upgrade for 2 stars the global upgrade total turns to 3 stars but the stars needed to get a new hero remains at 5. If you get a new hero the amount of stars for getting heroes still does not decrease. Diamonds are used to learn scrolls which changes your characters ability or to buy a specific hero (no spinner is used to get a hero with diamonds).
Tower Defense is a mode where you play as Garden Beetle (only available in this mode) and your goal is to defend against waves of enemies. You can not choose a second hero or any other hero other than Garden Beetle. As you progress the waves get harder. You can earn stars and gems in this game mode.