Rubber

IndustrialCraft 2 Classic (IC2C) has a tendency to use copious amounts of Rubber. Rubber, in real-life practical usages, acts as an "insulator" for cables (or, for a more correct usage, heat). Rubber has the same use in IC2C. Rubber is used to insulate Cables, preventing electrical "zapping" and energy loss. It is also used in a number of other recipes, and is gathered from Rubber Trees, a new tree added by IC2C.

Rubber Trees
To first gather rubber, you must find naturally-spawning Rubber Trees. Rubber Trees typically spawn in normal Forest (or respective Forest) biomes, but can be found outside these biomes. Rubber Trees can be identified by their famous Leaf Tip, averaging 3 blocks of Rubber Leaf Blocks. Rubber Wood (Rubber Tree trunk) spawns with "rubber taps", which can be collected using an Electric or Wooden Treetap. The taps will regrow over time, and do not spawn on every block. Alternatively, they can be cut down to gather Rubber Tree Saplings, which can be grown elsewhere, and produce the same Taps. There are many Rubber Tree farms, and multiple mods (including Forestry and EnderIO) have compatibility with IC2 Experimental Trees, which should work with IC2 Classic.

(Rubber Tree in Biomes O Plenty worldtype "ForestHills" biome, with multiple other mods)

Rubber Leaves & Rubber Saplings
Although not drastically important, both Rubber Leaves and Rubber Saplings have usage in IC2C. Rubber Leaves, gathered using Shears (or a Chainsaw on Shear Mode, default) can be used to create Plantballs, which can be macerated into Dirt or Compressed into Compressed Plants, which can then be used for the creation of Bio Cells (and ultimately EU generation), or Organic Mesh, which can, through multiple processes, be made into Carbon Plates, used in the creation of MANY higher-tiered machines and tools.
Rubber Saplings can, as any Sapling, be grown into a Rubber Tree. It can also be crafted into Plantballs, a TFBP Biome - Forestification, and extracted for more Rubber. As with Rubber Leaves, can be used for multiple other processes down the line.

Sticky Resin and Rubber
When an Electric or Wooden Treetap is used on a Rubber Tree, Sticky Resin will pop off. Sticky Resign is what you use to create Rubber. When smelted, will yield a 1:1 conversion, for one Sticky Resin will turn into one Rubber ball. Alternatively, you can place Sticky Resign in an Extractor for a 1:3 yield ratio, where one Sticky Resin will turn into three Rubber Balls.
Sticky Resign is also used in other recipes, including Sticky Dynamite, Organic Mesh, Leads, and a Sticky Piston.

Rubber, as stated before, is a very useful substance. Nearly every device requires Rubber somewhere in the line, and large amounts are essential in progression. Rubber is used in the creation of Copper Cables, Insulated Bronze Cables (1x and 2x), Insulated Gold Cables (1x and 2x), Insulated HV Cables (1x, 2x, and 4x), Quantum Accumulators, Nightvision Googles, the Hazmat Suit (Scuba Helmet, Hazmat Suit, Hazmat Suit Leggings, and Rubber Boots), alongside Rubber Sheets, various Charge Pads, the Rubber Boat, Industrial Belt, and various upgrades. The cables, especially, are used for many recipes, and the Copper Cables are required for the Electronic Circuit, a utility used in many craftings.